Was this strictly a verbal notification, or was it accompanied by official documentation (death certificate)?
If you have a death certificate, then you have all of the confirmation that you need, and should reject any subsequent ACH deposits in the decedent's name.
If you only have a verbal notification, I don't recommend rejecting the ACH. But to protect the bank, you could freeze/lock the account, accept the deposit and wait for either:
1) the death certificate, which will then prompt you to return the ACH deposit
2) the reclamation request
3) the depositor to visit the branch and let you know this has all been a terrible mistake (probably not likely).