You could contact the police: he has committed one or more of trespass (coming onto your property without permission, or remaining there after you--presumably--told him to leave), vandalism (posting signs on your property) and/or identify theft (claiming to be you in signing under your businss's name). While it's not clear the police will arrest him this first time, you can and probably should create a record of his actions; also, the police may speak to him to explain that this is not the right way to resolve disputes.If you suffer any loss (e.g. loss of business or customers) due to his action, you could potentially sue him for tortious (or wrongful) interference  with economic advantage to recover the loss.If the parking spots are all common spots, he has no right to restrict your customers from using them, though as a matter of courtesy, you might want to put up your own sign (written and approved by you) asking your customers to not park in front of your neighbor when possible. Doing so would also help you if litigation does not ensue, by showing you were taking reasonable steps to defuse the situation.