Ignoring all the arguments about freedom of speech:
That's like suggesting banning books and magazines on cars will lead to fewer traffic accidents.
Mankind has had war before these things existed.
The quest for power (and hence war) is part of human nature, and isn't suddenly going to disappear because there are no books or games or media or what have you on it.
I define power here as the ability to make others do what you want.
Also, banning things unilaterally makes them more desirable (and precious). More people will want them, and you put them underground where you can't see them and you can't see who is buying what. So, if your premise that the existence of the things encourages war is true, banning them in practice would only work against you. Also, more people will hear about them with you raising awareness and making lists of things that people can't have. The list would serve as a guide to the people who do want war.