I suggest an additional rule:
Copper Membership: are restricted to join campaigns if don't reach Member rank from earned merits (10 merits in minimum).
You know it's impossible, unless signature usage is tightly controlled by this forum and all service/manager are forced to use Content Management System (CMS) provided by this forum to run signature campaign. Is this what you meant?
Or do you mean Copper Member shouldn't get member's rank signature benefit and change it with something else?