Maybe a solution would be to make impossible to delete a merited post.
I think this can be easily implemented.
If the post is already merited and needs to be deleted, mods could also delete the accounts involved.
It's impossible to delete an account by forum moderator. They can just ban a user. I think merit abuse is not on the list of crime that punishable by ban? I found some merit abuse thread such as giveaway of merit for every correct answer for trivia question.