I don't quite understand what you mean by a badge for those that can wear signature. It's either you can or you can't, and as per the new rules, any rank other than newbie can wear signature (this also includes copper member newbie who can wear signature and post images)

And there are different ways of spamming, it's not a straight line, people can be clever when doing it and you won't be able to detect and same repetitive answers twisted over and again would lead to lead to a spam mega thread.
An idea would be to close a thread automatically when the discussion has gone stale and unproductive as users rarely return to their threads.

And I don't see why signature holders should be highlighted, ANY user spamming the forum is at fault, some are paid to bump threads without needing to wear signatures.

Report posts or comment you think is unhelpful or redundant and moderators would look into it, that's how we can put our hands together and maintain the standard of the forum.