Years ago, I participated on a forum where spam posting due to signature rewards was an issue, too.  Actually it was a pretty similar situation as it is on bitcointalk today.

After the introduction of a signature views measurement service, the campaign owners started to accept only members which had a high rate of views, mostly thread creators and creators of posts on popular and useful discussions with an interesting history. Or they paid rewards per view. Creating post number 1849 on page 190 on a pointless discussion thread, brought only a couple of views which finally became unattractive what caused an significant decrease of needless posts.

As far as I remember, it was not a native forum solution, rather a external service where a 1px image was added to the sig besides the text. Each time the pixel was loaded it counted the view. To prevent using visitor exchanges or other viewing tools, it locked the IP for a couple of hours to be counted for the same user.