Even though I am (obviously) on it, I think Danny's solution is a great one.  It's user driven, free market, and easily reversed if you want to see someone's posts again.  If more users adopt it then the signature campaigns will lose their value and die a natural death. 

While I think low value posts are the issue not the signature campaigns his solution is clearly working for him and I imagine would for many others.  It at least guarantees you'll never see another signature ad.