I'm tired so I don't know if this actually sounds as good as I think it does. How about max number of posts per week that can show a sig.
You can post as much as you want but only the 1st "X" number of posts actually show a signature.
400 posts this week. Fine, only the 1st 20 have a sig, the rest are blank.
-Dave
It's the people outside sig campaigns who come out with the most rubbish and nothing other than reporting them will deal with it unless there's some automated way of disposing of a post with several hundred Facebook and Twitter links.
Sig campaigns are gradually being tamed and are becoming more discerning. The threat of their removal is probably what's brought the grottiest into line. Dunno how you apply that to the content of posts. Then again the distant prospect of entering one is possibly what keeps shitposting hopes up.