Post signs that your parking lot stating it is "Private Property" and "Parking by Permission Only" or control the access via a gate or something, you are then (generally, varies by country and state) allowed to set parking however you see fit. Very many pay-parking-lots in my area do not have accessible spaces and are allowed to do this because they are not technically "public". (Yes, I actually asked city hall and that's what they said).
UNLESS you are also an employer in the US. In which case you must follow ADA guidelines and provide spaces based on the size of your lot and/or workforce size.
This is, of couse, all academic to argue about since once you are in a position to actually NEED those space (really, physically need them) you don't care how much other people are annoyed by them.