How about if "Ideas" was community moderated?
By default, new Ideas would be moderated. Until an Idea received some minimum number of votes (5?) from moderators, it would not be displayed to the general public. Moderators would be community members who meet some criteria, for example:
- Viewer is logged in
- Viewer profile is sufficiently complete
- Viewer has been a member for at least 60 days (or some appropriate number)
- Viewer has posted at least 3 forum messages in the past
This would practically eliminate the chance of spam getting through.
Also, a "flag as spam" feature to fast-track the elimination of bogus entries would probably help.