In a SNG it is hard to fight a maniac over-shover... the only way to fight them is with top hands... AA, KK, hands that are already made or top 4-5% IMO... AK is a good shove back at them, especially if you can shove it in before them, that gives them a moment of pause.

I tend to let these guys battle everyone else in the SNG, not changing my play much but getting tighter until they shove over and get knocked out... once that happens things return to normal.