Happens to me all the time.
If you block them in you are breaking the law, well if a traffic warden see's you he will ticket you as I found out to my peril.
Took a lot of negotiation to get him to take it off, lucky I saw him outside and ran out.
Blocking them in causes confrontations, anyone who is cheeky enough to park on someones drive is pretty verbose when they told where to park their cars

I use a 3/4 square marker pen to write on their windscreen some sort of "get orf my land" message.
Usually its some mum taking her kids to the dentist down the road, who drives off breaking the law with an obscured windscreen
