For a mere 100xp this power grants any wyrd some extremely cheesy close range options. Setting off a grenade hanging from your foe's belt? Sounds legit. Turning the safety on said foe's weapon or uloading its ammo? Why not. And these are only few options which can turn seemingly harmless power into lethal weapon.

How should a warband with psyker character aboard deal with this?

Easy, numbers. Any psyker with this can only target a specific number of things, one by the looks of it. That is potent but he cannot use it to stop a group of armed people from tearing him apart in the space of one turn, he simply wont have enough actions to kill an entire group.

Alternatively characters can find ways to cover themselves in protective wards/symbols/icons to shield themselves from the trickery of the warpspawn. Khorne aligned will find his attempts to mess with their gear very distasteful and will happily knock seven shades of... out of them.

Still this power is available from the very beginning, where hordes of foes or champions of khorne/emperor blessed enough to ignore sorcery might not be always manageble at all

Turning the safety on the enemy's weapon at best results in a lost Half-Action for the said enemy - the time it takes for them to ready their weapons. The only one of those that I would consider a problem would be to unload the enemy's weapon - this is something that clearly cannot be done in a single turn, nor be something that would be easily doable.

In both cases, the GM should arbitrate, but even in the best of circumstances, that's a pretty hard thing to pull off, so feel free to use that Hellish (-60) Difficulty, or require him to spend several rounds just shooting into the air - mind you, wresting it from the enemy's grip is either a Grapple or Strength Test, which the enemy in question can oppose, and if he doesn't wrest it from the enemy, odds are he'll either drop the weapon freaking out, or just aim it wherever he pleases - which he'd be doing anyway.

Setting off a grenade he can clearly see from the enemy is definitely legit, however. It's just as legit as having one of your other players aim their weapons at the grenade in question, and it should be pretty hard, so make use of those Difficulty ratings. Either way, it's a totally legit strategy. If it becomes a problem, just have the enemy start securing their grenades, or, again, require more than one round, since he has to grab the grenade, keep it steady, and then pull the pin, or pull off the protective cap and then pull the pin, or anything to that effect.

I also want to mention that Precision Telekinesis can be Sustained, and such should also totally legitimately be able to grab grenades being thrown at the players and throw it back.

Edit: Keeping in mind that Precision Telekinesis is a sustainable power, for many things like this, remember that it's subject to the rules for sustaining, which means that other powers will be reduced in Psy Rating and so on.

Some other things you could do is pull an enemies trigger before he is ready/ at a diplomatic meeting to cause havoc, nudge the barrel slightly, undue support straps/ belts ( seriously I have this image of a blood crazy Khorne cultists tripping over there own pant legs as the Tzeentch witch laughs) ect.

I do agree with fgdsfg that it should be a difficult test to affect something that small.

Thinking about it even if you just cause an enemy to lose a half action a turn turning the safety on it can still cause a lot of grief as they can no longer aim and shoot that turn or move and shoot that turn. Imagine if you are a sniper that has spent an entire round aiming for the bonus but then you lose all that by having to fiddle with the safety switch.

as for unloading a weapon, most solid/las weapons I imagine are like today where all you have to do to unload the weapon is hit a button and the clip pops out, so that could cause havoc right there and I'm sure it would cause a plasma or melta gun to misfire or jam if the ammo container was no longer fully inserted into the weapon.

and lets not forget vehicles, twisting the key to off while someone is trying to pull a high speed turn on there attack bike would defiantly cause trouble.

Remember that you can only do such things you could do otherwise. And it's not that easy doing such things otherwise, which would translate into the actions using PT.

Remember that you can only do such things you could do otherwise. And it's not that easy doing such things otherwise, which would translate into the actions using PT.

Well that's a truth some some modification to it. Yes, you can only do such things you could do otherwise. But at the same time, you can do things you would absolutely never be able to do without fearing for your life.

Htsmithium mentioned attack bikes, so I'll stick to that - imagine slamming your whole body into an attack-bike as it's getting close to you. In person, you'd die. With Precision Telekinesis, you can do it without any problems at all.