FYI a work colleague, motor vehicle assessor, told me that my R33 GTST 1997 S2 is worth $16,000 market value under the company I work for... Not bad for full comp @ <$800 a year premiums... xs $1150. Yes they insure WA under a couple of brands.
Sounds like you have a problem dude, I'd say maybe do a stake out, do they come during random parts of the week, weekends? If you can get the plates you can report them to the police. Good option is parking your car in the garage and parking another car in the way though.