Ask an Property Solicitor. Get an Answer ASAP.
You can put up a sign. Are you experiencing problems with your land and people parking in the space?
If you own the land then the neighbour is committing trespass by parking on it.
You can put up a sign to inform the car owner that it is your property and not to park.
Do you have a copy of the title deeds? I can check them for you if you wish to confirm the position further.