I'd like to see signature only shown if that user received X merit in last Y days. I don't know appropriate value for X and Y, but i expect it could somewhat reduce spam.
Let's make it 5 Merit in 120 days, which is what some campaign managers ask as a minimum to apply. That suddenly makes those 5 Merits worth 17 weeks of campaign payments.
Aside from what @UmerIdrees said, it's likely campaign manager will choose someone else that have better recent post quality or at least receive more merit in past X days.
This will lead to Merit abuse.
You're right, there's always trade-off of any changes on merit system.