Can I prevent power theft?
It's difficult to completely prevent theft. The illegal tap could have been installed long before your presence. Such taps can be nearly impossible to locate. It's even difficult to prevent new taps from being installed because meters and overcurrent devices need to be accessible and you cannot reasonably monitor external wiring 24/7.

While it's difficult to stop, it's not too difficult to detect, though somewhat inconvenient. Turn off all the branch circuit breakers in your unit, then inspect your meter. If the meter indicates current is flowing, you are paying for power you are not using. Of course, if you run this test when the thief is not using your power, you cannot detect it. You need to catch them in the act so to speak.

Also, if the thief has tapped into your branch circuits, you have also shutoff their access so they are undetectable. To identify this kind of theft, you need to disconnect every single thing in your unit that uses power. This includes doorbell chimes, smoke detectors, clocks, furnaces, stoves, etc. Then once again check the meter. If the meter indicates current flowing, power is being consumed that is not under your control.