I hate this type of behaviour - dicking around on "the edge of the rules" to annoy people deserves a sound beating. Assuming your not up for doing that, I think you need to fight fire with fire.
Step 1 - Install CCTV
Step 2 - Every time they do something dickish, post it on youtube (So no one can claim only you had access to images in step 3)
Step 3 - Every time they do something dickish, you do something dickish, leaving an image from the CCTV with "this is why" in place. Its really up to your imagination what you do - the trick is to be annoying without being illegal (Or at least not SO illegal the police would be interested). For example, smear something unpleasant on the windscreen, take the wheels off (Leave them there, just take them off the vehicle), stuff like that.
It may be that its a quick win. If they are the type to fight a war, this may result in them escalating, breaking the law properly and being removed.
Not an ideal scenario, but it looks like they are staying close enough to legal to make "formal" steps impractical.