I'd let the landlord know immediately. Ask the landlord to do something, like a lock box, to protect your outlets in the common area so that this won't happen again.

In the meantime, if you have access to your circuit breaker box, you can flip the switch for that outlet so that no power gets to it. I had to do that one time and the neighbor never did figure out why his washer and dryer suddenly stopped working.

It's rotten that this guy stole from you, but I'm not sure there is anything you can do to get the money back--you'd have to prove he used your electricity and how much.

Technically it's theft and is a crime. I would definitely report it to the landlord.