As I posted over on Landywatch - you don't want to get in a war with some of these people! - they know where you live, you don't know where they live.
If you ramp up hostilities beyond a certain point - they could torch your car... or worse!
Bear in mind that, if they are nicking these vehicles primarily for profit (as against joy riders), then the main defence is to address the risk benefit ratio - make it harder and possibly riskier, to make a profit.
Apart from making defences harder to get around, another idea is to consider the possibility of civil legal action (in addition to any criminal action that the police might bring), since the burden of proof is lower.
Of course, with villains, they are never going to pay £thousands in damages, but it allows their names to be published, and bailiffs can torment them long after some nominal custodial or suspended sentence. It all helps to take the 'fun' out of their business
cheers