I worked underneath 3 PokeStops. One was a gym. Used to troll the EVERLOVING JESUS SHIT out of the gym. Great way to spend a break/lunch and let off steam at the same time.
Go in, beat the Pokémon in the gym. INTENTIONALLY leave it unattended and blank. Then, when the next sucker, errr....trainer, put their Pokémon in, I’d beat THEM, too. Usually, this is done with a lower level Pokémon, just gaming the type they put in. Rinse. Repeat.
Then, if no one takes it within about an hour, I’ll usually put in a Snorlax, Chansey, or Salamence to dick with everyone.