Tomorrow, why don't you go to your bank and stop all direct deposits into your account from the specific former employer? You and you alone control deposit allowance into your bank and retirement accounts with the bank.
Do your demand to block deposits in writing.
All deposits that you have gotten that you were not entitled to get will have to be returned. Clear this up tomorrow. If done in the same tax year, you will have no tax consquences if you accurately report the situation.
Good luck. And, I hope this helps.