My first step would be to find out if their is 1 or 2 circuit breakers in your power box that supplies the granny flat by switching them off in turn. If their are breakers just for the granny flat wait until Friday night then switch them off, having previously bought a special lock (that the power company can unlock) from a locksmith that you can use to lock the power box.
See how he likes being without power for a whole w/e.