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).
This has nothing to do with Meta, I think it should be in
Reputation.
Back ontopic: I've
done that: I accepted Copper Members equal to Members, as long as they earned 35 Merit.
This has nothing to do with "being unfair", all users can buy Copper Membership if they want it. Campaign managers shouldn't pay spammers though, with or without Copper Membership.
Copper Members can get their capital back after ~ 2 months; they won't lose nothing, and they can keep spamming.
Click "Report to moderator", they can still be banned.