Could work too, but removing their ability to wear a signature would be enough in my opinion because it removes the motivation to spam.
Also, they would have the opportunity to improve the quality of their posts and ask the tagger to remove the tag, or others to untag him.
If the user is banned then he can't improve because he just can't post.
Or maybe it could have 2 levels depending on the number of people tagging him, the first one would remove the ability to wear signatures and the second one would ban him.
Hmm I was thinking something along the lines of warning/temporary bans rather than immediate permanent action.
I guess my issue is that a good deal of users might try to build up post/activity count to join signature campaigns later, despite having their signature enabled.
Actually, since the people causing this issue usually just try maximizing metrics (post count/activity)... I wonder if freezing those could work too.