I never sat through anything anti-MJ at uni. My freshman year there was a large lecture on the dangers of MDMA use but that's about it.

Personally, I think trying to argue with a speaker if it does start going south is the wrong thing to do. Unless you're prepared with what you're going to say and can do it without making an ass of yourself it might do more harm than good.

I think it would say more for you and your posse to just get up and leave. Inform the speaker why you are leaving so everyone knows. Tell them you won't sit through a propaganda-filled sermon and are only interested in facts. That'll go a long way.