Earlier quoted context omitted.
> Another solution is to hide all links when a page is viewed by a user who isn't logged in. No, this is almost as bad as what medium.com does with all its links (rewriting them to point inwards towards redirects for tracking purposes). A web link should be respected.
> (rewriting them to point inwards towards redirects for tracking purposes) I like this idea a lot. It has more than one use. Tracking is one but eliminating the "PR bleed" is another and it should deter spammers who are looking for link weight. I understand the sentiment you're conveying but a link isn't sacred. If you allow users to post content to your site then you should be taking measures to ensure that new use…
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.