If the links are to legitimate sites which you want to be associated with, I wouldn't add nofollow to them. Outgoing links are also considered part of the algorithms which search engines use to determine rankings for this site.
I run a forum where outgoing links are sometimes added by the forum members. This forum software uses a plugin which controls the use of nofollow on outbound links. The configuration I use is that all links are nofollowed by default, but that domains which are relevant to the site can be added to a white list. Periodically I scan all new posts for new links and check for sites worth adding to the whitelist.
If you use a well-known CMS or forum package for your user generated content you may want to search for such a plugin. If you have the coding skills, you could also add such a functionality to the code yourself. In that way you have the advantage of relevant links counting in the SE ranking algorithm, and you are at the same time protected against misuse.