Post signs in your driveway saying it is a private driveway. No trespassing signs would be good, too. You can actually call the police on trespassers then, if you want.
Signs are as good as the material they are printed on. I find that the best way to keep people out of my driveway is by erecting a chain or gate across the driveway that you have a key for that people cannot remove or drive around.