those merit abusers that will be caught can received Negative Trust so they can still received enough punishment.
The Merit sender has red trust already, so he doesn't care about more red trust. The Merit receivers can't be blamed, as they can never prove their innocence. That's why I think a demerit is the best solution.
Not bad, but abuse can also be avoided by raising the merit requirement. Theymos suggested up to 5, but it could be even up to 10, and then change member requirement to 20 or 50.
Even though the amount of Merit required on each rank increased, there are still a lot of possible merit abuse out there and there will be more. Can you imagine giving 50 Merits to a garbage post? 50 Merits to a one liner posts? It's a total abuse. So deMerit is the best solution.