[from the carnival game which involves quickly and repeatedly hitting the heads of mechanical moles with a mallet as they pop up from their holes.]
1. The practice of repeatedly causing spammers' throwaway accounts and drop boxes to be terminated.
2. After sense 1 became established in the mid-1990s the term passed into more generalized use, and now is commonly found in such combinations as whack-a-mole windows; the obnoxious pop-up advertisement windows spawned in flocks when you surf to sites like Angelfire or Lycos.