It should say in the deeds whether they can park on it or if it for access only. I saw something on TV about a similar situation years ago. One neighbour would park all over the drive leaving the other with little space. He felt he was entitled to do so as he had a share in the whole drive, not half each with an imaginary line down the middle.
Have you tried talking to them? Perhaps they don't know they're doing anything wrong. Check the deeds, anyway.