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.
A little extreme as a post which is worthy of deletion doesn't always mean that the account should be banned/deleted. I think the better alternative is that once a post has been deleted the merit no longer is credited, and the merit point either gets lost in limbo or refunded back to the original sender.