Got overbilled by IHSPM, they won't refund

These guys are the worst I've had to deal with in the property management space. I was getting overbilled by IHSPM for months and getting the issue corrected wasn't easy either. They acknowledged their mistake and corrected the subsequent invoices, but now to get the refund for the previous invoices is a total nightmare. They (IHSPM) have simply ignored my communications. I was communicating directly with the property manager & accounts people. My next option is to take them to court to recoup my money. My advice, run away from IHSPM like a plague