I've considered rewriting user links to scramble the text and point them inward at a link proxy (which then routes out through an anonymizing service like anonym.to). Anything to destroy their value.
Haven't actually put it to practice yet though and I wouldn't be surprised if it wasn't a good idea. Unfortunately it seems that if spammers catch just a whiff of SEO from you, they'll never stop, like cockroaches.
[edit] I also haven't tried putting it into practice because I can imagine how utterly annoying it would be. I can't stand those "you are now leaving [site x]" pages myself.
I'm not a fan of those either. If you want to avoid having a "now leaving" page while still deterring spammers then you could create your own URL shortener with a separate domain. The link text could be the original link but the URL is the one generated by your shortener. You could even restore the original text with a replace() if you ever felt the need.
Haven't actually put it to practice yet though and I wouldn't be surprised if it wasn't a good idea. Unfortunately it seems that if spammers catch just a whiff of SEO from you, they'll never stop, like cockroaches.
[edit] I also haven't tried putting it into practice because I can imagine how utterly annoying it would be. I can't stand those "you are now leaving [site x]" pages myself.