Remove nofollow from links which the user posts
It's to be hoped that the user posts high-quality, relevant links.
If they begin to spam there are other (more severe than adding 'nofollow') ways to deal with that.
See Remove nofollow on links deemed reputable
See also Can we not rel=nofollow links in profile About Me for users with 3000+ rep?
See also Stackoverflow amongst nofollow web abuse sitesStackoverflow amongst nofollow web abuse sites (caveat: this seems to me an interesting point of view on this subject however I don't know whether it's true).