Let's ban knifes.
Seriously though, I do believe violence can affect the minds of people. Yes, violent conducts can arise from the continuous viewing of violent actions, and yet this same violent conducts can be repressed through education. I play a lot of violent games, and I am not violent. Well, maybe verbally aggressive sometimes (Not as in screaming bad words, but as in glaring and puting some kind of deep evil voice saying nothing polite) but certainly I would not likely move to physical violence unless taken to a total extreme, and still not even then do I believe I could hurt someone much, with that I mean I would give the person a few punches but I wouldn't move beyond that (Well, such has been the experience the only two times in my life I have attacked someone who has made me royally pissed).
However, people of short intelligence and wisdom or short sanity levels can get driven to do violent actions. However, videogames are just a little factor on the whole collection of factors that may lead to this. Will you ban bad education? Will you ban loneliness? Will you ban genetic factors? Will you ban the fact ours is a damn violent society? Of course videogames are a factor that may lead to violent actions in SOME people, but they are not the only damn reason. Someone normal dosen't stab anyone because someone in a videogame does so. And someone normal can't get "unormalized" by a game.
And Manhunt sucks, anyway. I got it for free from the videogame site I work for to give it a review, and I had to play all the damn game, it was so terribly repetitive...