I don't think Theymos has reversed any merit grants yet, at least none that I know of.
It has been done once as per LoyceV although I don't know who was the person, whom he sent merit.
I've only seen theymos undo it once, when a hacked account had send out 3 times 50 merit.
Only @LoyceV can tell who was the persons. AFAIK, LoyceV is using piggy's notification bot. And it will soon redirect him here to share the usernames of the first reverse of merit transaction.