Gas cap Lock?

It's time to attack the problem instead of the result.

Several things you can do. Set up video cam, alarm the car, restraining order, charge him with trespassing, theft, and get a pocket audio recorder. If he makes ANY audible physical threats, get the law involved.

If that doesn't work, Wilson makes aluminum baseball bats. Come to think of it, wooden ones work, too.

I have a locking cap, but the key is manufactured by Ruger