In my experience with web forums, this is the principal problem with trolls. It's kind of like real life, no one wants to be around them. So after a while, you end up with just the trolls on your site.

Maybe you can monetize trolls somehow? But I've never been on a team that came up with a good way to do so. No one wants to be 4chan.

In the end, it's always been better for the bottom line to try to make them go away.