Start billing them for your time taken to correct their errors.
Make sure you send them a notice that you'll be implementing this policy, giving them time to alter their practice of using you to solve their inaccuracies.
Wonder what would happen if you pay only what you know is correct? Would they take you to court? Slap a lien on you forcing you to take them to court? In which case I wonder if you would win costs agains them?