Yes, you can. However, it is best to spell out in your lease agreement the events that could cause their car or truck to be towed. You'll also want to include information about who is liable for that expense. In most areas, all you must do is show the lease agreement to the towing company to provide permission for the tow. You might also include contact information for the towing company in your agreement. If your parking lot looks like public parking, you'll want to consider installing signage indicating that it is private parking only.