I always thought Holocaust deniers were irrational in the first place (at least, that seemed like about the only way somebody could deny the Holocaust and keep a straight face; if they were off-balance in the first place).

I don't know about a safe place, but I'm not sure I see the point. I agree with LH that they basically do it to hear themselves talk and get attention. Know your facts, but don't encourage them. They're squeaky wheels and the more they squeak, the more likely that somebody is going to hear them and latch onto their ideas even if you're arguing rationally against them at the same time.