What is the name of your state (only U.S. law)?
Minnesota
Details, please.
What kind of services were quit, and for what reason are they demanding a refund of their fees?
If they were dissatisfied with the services you provided, they may have a case against you for reimbursement of service fees.
If they quit for their their own personal reasons, and the services were provided as promised in your service agreement, then they might not be entitled to a refund of those fees.
Either way, we can't give you a better assessment without knowing the facts.