Amend the forum to hide signatures of any account that hasn't earned at least 1 Merit in the last 2 months.
This will bar them from getting paid to post and not have any negative impact on anyone making even the tiniest contribution to the forum. Nobody loses their rank and even if someone is away from the forum for a while they can quickly get their signature back.
Hah! Easy for you to say... What about cute trolls like me that people seldom give merits to? I sure am a narcissistic assh*le in the forum at times, but I'm far from being a spammer.
And I'm not here to please anybody for merits. F*ck that.
One way to make this forum better is to be extra stict for a year or so...