How can I stop the repo man from taking my car?

Firstly, it is not "your car" ... when borrowing money to finance the payments on a car, the lender owns the car ... you are just the registered owner. You own nothing until the final payment has been made.

IF you default on any payments during the payback period, the lender then assumes you no longer want or need the car, pure and simple. So, they repossess the vehicle, cancel your loan, keep all monies you paid to them, and then resell the car to somebody else. The lender is entitled to take whatever legal actions are necessary in order to recover money lent to a borrower in good faith that they would make the monthly payments.