They don't have to "honor" the divorce decree. That's between you and your husband, and does not change your contractual obligation to them.

Can you prove in any way that you closed the account to further charges? You may be able to avoid responsibility for charges made after your ex- reopened the account.

You have the right to tell a third party collection agency to stop contacting you.

You can pursue your ex- for the debt in court. If he's hiding, you can ask the court to authorize substituted service.