How do I stop AirPods from being tracked by the previous owner?

You cannot. Unless the original owners remove those AirPods from their account, you will not be able to use them. AirPods can only pair to one Apple Account at a time. And only the original owner can remove them from their account. Ask for a refund.

You don't. The previous owner did not properly prepare them for sale. Return them and get your money back if they will not do so.