We had a similar problem on http://www.rubyflow.com/ - a similar Ruby community site. Our solution for a long time was to only include items from "approved" users in the RSS feed. This worked very well but then people got annoyed they weren't added to the approved list and we didn't have enough mods to catch up with it.
So now we have the RSS feed on a 3 hour delay from the live site which means spammy posts are often reported or moderated away before they hit the RSS feed. It's not absolutely ideal but works given the circumstances. To create such a delay involves only the most basic of tweaks to the query that fetches the items for the RSS feed.