Firstly I want to say they are being really unreasonable and I would go mad if my neighbours kept doing this!
But, I've always been led to believe that it's not illegal if your drive is empty. It's illegal to block you from getting out but not if the drive is clear. I could be wrong but I'd double check that before you take it too far.
Hope I'm wrong and you can stop them!
My understanding is the same.
It's illegal to block an occupied driveway, however if your car isn't there then it's not illegal and the police won't do anything.
However, it can't help putting a call into your local station to ask their advice. Call them direct, don't use up 101 time.