The one time only (so far) merit reversal was for a case where Elwar apparently has his account hacked for a brief amount of time (fell for the .to phishing site), during which someone sent 3 x 50 sMerit TXs from his account to threads he has never visited. The thread where the case is discussed is this one: Showing I merited 3 people 2 days ago but I never even visited those threads.
Post #25 on the referenced thread is @theymos' answer to the case, and post #29 delimits options of that action extending as a norm.