Give people the tools to opt in and out of whatever filters they please. Even turn it on by default if you want. This is one of the few things Twitter has done right - you can simply turn off their "quality filter" if you don't believe that they have your best interests at heart when deciding what shows and what doesn't. (Where Twitter does wrong is a number of other opaque filters that cannot be disabled)
Reddit has a simplistic version of this. By default, you will not see anything that has a score of -1 or below.
Because people don't like to participate in a platform that has gone to crap. It's happened on many sites/forums in the past. If you don't weed, you won't have much of a garden.