Your bank has a contract with Husband and Wife. No two of you can decide to change the terms; i.e. he cannot take her off, she cannot take him off. Yet, either of them can close the account. So, whoever wins the "race to the bank" gets to close the account. You are in no way responsible for taking over the administration of outstanding checks, recurring ACH transactions, etc. The account is simply "closed" and that's the response you will provide to those petitioning for payment.
Some banks have a form where all three parties purport to "amend" the contract; e.g. she can "agree" that her name can be removed from the account. In the days when we inspected every item before payment (the previous century) it was an okay idea. Today, when banks pay on the MICR line and do not inspect the items, it is an incredible, somewhat ridiculous assumption of uncompensated risk. If she's been taken off the account and, because he's past due in child support, writes herself a check (one she kept in her purse for emergencies) your bank is simply going to pay it.
If she signs her own name, it's definitely not a forgery and you're going to have to get the money back from her. He's simply not in the picture because your bank agreed she was no longer an authorized signer.
If you have such a form, go to every branch and pick them up. As they epitomize the lazy way to do things, your personnel will not give them up voluntarily.