I asked if there were rules on how to do this and the conclusion (at least so far) seems that there are none.

Which means the rules have another glaring hole we need to fill. Because when that big bad sorcerer boosts himself with Hammerhand, Molten Man and Flesh like Iron you want to be able stop those boosts before engaging him im combat.

The rules I propose work as follows:

1) The simple, fast method: the threshold to dispel is set by the casters manifesting result (i.e. you want to manifest a power with threshold 15, roll your dice and get 21 = dispel-threshold is 21). Hard to dispel powers like this but little extra dice rolling and using more dice hace the added benefit of making your powers hard (and risky) to dispell.

2) Both psykers involved roll their dice. Could be done either with willpower or manifesting (psyrating etc). However you have to decide what kind of action it takes to keep a power you manifested from being dispelled.

Those are the two ways i thought about. Other things to discuss are what kind of action is needed to dispel a power, do you need to know/recognize the power to dispell it and is there a difference between sorcery and "normally" manifested powers and is dispel an action or a talent/power you need to learn?

I eagerly await your opinions.

Generally speaking, the background hasn't shown that psykers are capable of countering enemy psychic powers on their own (it used to be possible in 2nd edition 40k, and is allowed in a limited fashion in Inquisitor, but in those rules a character can only nullify powers used against himself). However, we know that there are devices that help in this manner. Below is one such example:

Psychic Hood

These rare devices are used most famously by the Librarians of the Adeptus Astartes. Their purpose is simple - a psyker so equipped can calm and quell the surging tides of the Warp and halt the manipulations of enemy psykers and sorcerers. Requiring extensive cranial surgery to fit, psychic hoods are often mounted in helmets to protect both the psyker's head and the delicate circuitry within the hood itself.

A character wearing a psychic hood may, as a Reaction, attempt to stop a psychic power he can perceive from manifesting. This requires a power roll with a Threshold equal to the enemy Psyker's power roll. If successful, the hood negates the target power's effects and prevents any psychic phenomena that might otherwise have been caused. As the Warp is calmed by the use of the hood, there is no risk of Psychic Phenomena when using it to stop an enemy psyker's power.

There is no maximum range to a Psychic Hood's use, as it influences the Warp where time and space flow differently, if at all. However, the practical limits of its range are a matter of the wearer's ability to perceivethe efforts of enemy psykers. It requires a Psyniscience Test , with difficulty determined by the GM, to discern that an enemy psyker is using a power. At close range, this will be simple, but at longer distances, it may be virtually impossible.

Psychic Hoods cost 15,000 Throne Gelt to purchase, are Very Rare.

Another useful device, posted onto the old forums by TS Luikart (who wrote parts of Dark Heresy, including the Psyker rules) was the Null Rod:

NULL ROD

Ancient devices that only the Tech-Priests of the Machine God still have the ability to produce, null rods are powerful anti-psyker weapons, but exceedingly dangerous to wield for extended periods. Crafted exclusively from a rare form of obsidian, activated null rods blaze with a corona of negative-psychic energy which all but shuts down the connection between realspace and the Warp. Within 3 metres of an activated rod, no psychic powers of any kind function and manifested daemonic entities can only resort to physical attacks. This blanketing of psychic energy cannot be excluded and applies to everything within the 3-metre radius, including psykers allied to the wielder of the null rod, or even the wielder himself. Note that the dampening only applies to directed psychic effects that start within or try to move into the field. For example, a telekine that hurled a Force Bolt at a null rod wielder would see the energy of their attack harmlessly dissipate 3 metres away from their target. If the same telekine picked up a chainknife with their telekinesis and hurled it towards the holder of the null rod, the whirring knife would likely still be sailing through the air due to momentum as it hit the 3 metre ‘barrier’ of negative psychic energy.

The reason null rods aren’t wider spread isn’t just due to their extreme rarity or difficulty of upkeep, but the extreme danger implicit in how they achieve their means. The Magisters of the Inquisition believe that psychic ability resides within the chemical-electric pathways of the mind, within the dance of neurons that make consciousness possible. The minds of psykers have far more of this neuro-energy which null rods seek to dampen. The diminishment of mental energy around a null rod is most notable in psykers, but actually occurs in all beings exposed to a null rod. Non-psykers exposed to null rods for extended periods begin forgetting important pieces of information, eventually followed by losing consciousness at random intervals, and followed by permanent brain damage as parts of their mental faculties are shut down due to the slowly corroding influence of the rod. This eventual baleful influence is well known to the Ordo Malleus and few dare to wield null rods in any but the most important battles.
Every five minutes spent in the presence of an activated null rod causes all within its aura of influence to successfully take a Routine (+20%) Will Power Test or acquire 1 Insanity Point. The test is a Challenging Will Power Test for the null rod’s wielder.

Stats: Dam: 1d10+2 E Pen 3 Spec Qual: Power Field Wgt: 1.5kg Cost: 12000 Very Rare

Hanging around with Untouchables (see Disciples of the Dark Gods for rules) is also good for blocking psychic powers.

I too am a little wary of introducing 'dispel' mechanics. Those around in the first installment of Inquisitor for example were disasterous; more than one psyker on the table and anyone trying to do anything would lead to a lot of psyker-brains dribbling out of ears.

With DH, however, I have an idea for a slightly intriguing 'battle of wills' approach. That is: you add the psy-rating of an 'opposing psyker' to any power's threshold. Quite how to manage the 'who counts as opposing and how many times can you do it' is something I've only tentatively thought about and haven't arrived at a sensible solution yet.

There's also, IMO, the effect that simply having many psykers in one place ought to make things more turbulent. A mechanical manifestation of this would be widely applying the 'weaken the veil' power any appropriate time when an abnormal plurality of psykers arises.

One might imagine a discipline of powers in which a psyker can train himself to oppose another. What to call it, what mechanics and how to describe it I honestly haven't thought about, but it does seem a sensible discipline and one which I suspect players might feel thankful for.