If I have reached draw cap can I kneel Tommen Baratheon to draw zero cards in case I just want to kneel him?

Short answer: yes

Long answer: Why would you want to kneel him if you're getting no benefit whatsoever?

There are a lot of situations for this. The simplest case is using a plot which says that "knelt characters cannot be killed".

Flintacs said:

If that's what you wanna do it, sure. I personally don't use that plot cause it gives your opponent the benefit as well… I'm not a big fan of giving my opponent a leg up in any way.

Is it really allowed? When you can't do the action you can't pay the cost for it, can you?

Flintacs said:

There is no rule that says "in order to trigger an effect or ability, you must anticipate that it will resolve successfully." Quite the contrary. Instead, the rule is that "in order to trigger an effect or ability, you have to be able to a) meet the play restrictions; b) choose all required targets; and c) successfully pay the costs." So it's all about whether or not you can successfully initiate the ability, not whether or not it will successfully resolve.

The Draw Cap does not create a play restriction on initiating effects, so it doesn't interfere with initiating Tommen's ability. Once you successfully initiate it, the rule stops it from resolving successfully (we usually say "it fizzles"), but by the rules, so long as you can meet the play restrictions, the target requirements, and pay the cost, you can initiate the effect, even if there is no reasonable believe that anything useful will happen when it resolves.

Thank you! Now it's clear.