You can always look at the 'karma' system-- it could be modified to do this.
But honestly I'll give you my easy solution to your deletion/free speech worries. Create a 'junk' forum. Mine is called The Shark Tank. Whenever someone posts crap, the thread is moved into the Tank. If people want to read crap, they go there. If people post crap, they know it will end up there. Problem solved