Anywhere between the place you're trying to de-power, and the place supplying the power, is a place you can disrupt that power supply. Bits of equipment like fuse boxes/breaker boxes and transformers are easy enough to alter in a way to cut power to a certain building (or set of buildings) if you know what you're doing.

Also this is technically not legal advice, but please do not try and mess with your power grid in order to do crimes, it's most likely another felony charge or two if you do it.