Half-baked idea: Combine loud classical music and CAPTCHAs!
I have half-baked ideas from time to time. This is one of them.
It seems that some places use loud classical music to keep certain types of people away. There's a liquor store in Sacramento which claims victory over loitering by cranking loud classical music outside the store.
Web sites which allow comments frequently use CAPTCHAs to keep certain types of comments (spam) away. While they might slow down spam, they don't usually stop the humans who ruin social environments.
Maybe it's time to merge these ideas: have audio CAPTCHAs in which certain kinds of music are played. The key is to find the kind of music which is truly hated by the evildoers.
Then you just embed a few digits into that recording and require they be entered before allowing the post.
Just remember, you saw it here first.