I think they can prevent you from registering the following semester. Regardless, you're parking on their private property so if they show you have a grip of unpaid tickets then they will just tow your car away.
I gave up paying for a parking pass and would just keep recycling parking tickets. I would have to get 11 tickets to pay for a pass, so every time they ticketed me I would just park again someplace convenient and put the ticket back on my windshield. I usually would rack up 8-9 tickets, but I felt like I was sticking it to the man.