I don’t pretend to know about SEO, but unless the user is tl3 then the links should be marked nofollow. So shouldn’t they be ignored as far as SEO is concerned? (but I think I saw somewhere that nofollow was more of a suggestion than a rule).
If they always use the same one (e.g., xyz.net, which I have never seen anyone use before in places that I have seen) you might be able to use watched words to replace them.