Not agree with one merit, it can be bought or trade from corrupted member's. Remove signature is the best solution as you say. My opinion just leave merit system same as now , just disable signature campaign for up to Jr. Members. If requirement small merit, merit abuse will increase. We should try to prevent merit abuse as well.
Well you have a point there. Removing signature would make less merit abuse on that rank, but i think the solution where managers make restriction for Junior Member to join any campaign is a great idea too or at least making a minimal merit requirement for Junior Member to join a campaign. Three of them is good anyway. We just need it to be applied as soon as possible.