I would propose that if in the space of 30 days (rolling) you have more than 10 posts deleted for spam, then you automatically receive a one week ban. This increases to 2 weeks, 1 month, 3 months, permaban, on subsequent offenses. (Replace numbers as you see fit.) Every signature campaign that has a member banned for spamming receives a negative point, and after x number of negative points that signature campaign is booted from the forum.
The one reason for the merit system to be implement was to remove the spam as the spamming accounts won't be able to rank up and those already ranked up will get no current merits. Most bitcoin signature campaign require you to have at least 5 merits in last 120 days.
Even then we see the spam from those poster who have gained the minimum merits requirements. Now whom to blame this?
I know Altcoin campaign allow Jr members / Members and no merit conditions but lets just try to first find the root cause through bitcoin signature campaigns.