You need a good alarm system. My suggestion: Glock

I've had to stick a .40 cal in a guy's face before during a situation similar to that. This was off of the BP gas station at Clairmont and the guy got IN my car (via the passenger side) while I was still in it (I had the driver's side door open while I was letting it cool down - my own turbo timer if you will as a normal turbo timer for my car de-activates the alarm system - that's a no no). Put a .40 cal in their face before they can even breath and that will teach them to stop VERY quickly...