The key is fast moderation by folks who are knowledgeable about the area they are moderating, then whacking junk from past if it bounces to the top for some illegitimate reason again.
Unfortunately the system does not lend itself very well to promoting quality content either, so moderators have to battle with the system as well as reduced moderator functionality since the new JIVE version.
I am personally convinced that SAP's own hosted community, interaction with real gurus and the possibility to become or show subject matter expertise here is SCN should be more than enough attraction for the correct professional behaviour of contributors.
The childish points system and ill thought out ways to keep it alive is the root of almost all the problems...
Cheers,
Julius