Not a lawyer, but here's my take.

I am assuming this was a joint account. If that is the case, hopefully you canceled the account in writing and/or instructed them to remove the ex from the account.

If you didn't do it in writing, it is your word against theirs and you are obligated to pay the bill.

If you did submit in writing the ex is no longer authorized to use your account or that you were closing the account, dispute the charges as you obviously did not authorize them.

The bottom line in this is where the liability is. You gave the ex "full authority" to use the account accepting responsibility for his actions. Unless you have documentation stating otherwise, it is your word against the lenders of whether or not that phone call took place.

Always ask for it in writing. What you can do is try to negotiate a lower payment, or settlement on the total but be careful how you negotiate it. Tell them as part of the agreement, you don't want adverse information on your credit report.

Before you pay, get the agreement in writing.