I think I've heard that they prefer to remove things like that algorithmically, rather than writing hard filters for individual sites. Makes sense, because if you can come up with an algorithm to get rid of known crap, it will probably also get rid of some unknown crap too.

Yes, hard filters at a place like Google would quickly get out of hand.

Not to mention probably calling down the wrath of the FTC for anti-competition violations.

Just create a list of banned domain names. And let the users do that as well.

You could do both at once, actually, so that users aren't subjected to the junk results.