Action Comics # 273 (Source DC Wikia)

4. Namrepus

Mr. Mxyzptlk is one of those classic silver age Superman villains who struggles to exist outside that context. Mxyzptlk was an all-powerful imp who used his powers to annoy Superman a bunch, only leaving him alone if Superman could trick Mxyzptlk into saying his own name backwards.

In Action Comics #273, however, Superman decides to turn the tables on Mxyzptlk and be a massive jerkface right back at him. Superman travels to Mxyzptlk’s home, the Fifth Dimension, and finds that the imp is running for mayor. Superman systematically ruins Mxyzptlk’s mayoral campaign at every single turn until Mxyzptlk has had enough. He decides to make Superman have to go home by tricking him into saying “Namrepus” (“Superman” backwards), and actually tricks the Man of Steel into doing so.

Unfortunately, Superman’s real name is not “Superman” so it doesn’t work. Superman sticks around until Mxyzptlk has definitively lost the election, and then leaves of his own accord, satisfied he has ruined Mxyzptlk’s life for the time being. Because Superman is Superman, and Superman is a jerk.