Dave Winer suggests to solve the comment fraud problem by adding expiration dates to comments:
Here’s a free idea I had the other day while cleaning up a spewage of comment spam. What if comments, by default, were deleted after 24 hours? What if the owner of the site had to check a box in order for a comment not to be deleted? That way if a comment had lasting value, the owner of the site could make sure it sticks around.
Interesting idea. In fact this would be easy to add here. But until now all comments on this weblog are worth keeping. So maybe a “expire this comment” checkbox would be even better.