He should identify if other accounts are paid by MB or SS to upvote a post. He tracks all incoming transaction to these vote selling services. So if someone sends - let's say - 1 SBD and the post link as a memo to MB, @trufflepig will track this purchase of votes and discount the reward accordingly.
Maybe I didn't really get what you mean, but does this answer your question?
Someone can sell an upvote to smartsteem or minnowbooster and keep their SBD credits in SS or MB wallet, then use that credit to upvote with no transfer hence no memo trail
Ouch, yeah this is almost impossible to detect :-(