You are not required to let the entire Yokum family run all of their income tax refunds through Mammy's account. Repeating what others who know more about ACH than I do have said here, the return code is R03 and is used when the account number is valid, but does not correspond to the individual identified in the entry. I suggest you let them know your plans in advance.
Simply indicate you do not allow ACH credits in the name of anyone but an account owner. If you say it, it's your policy. It does not have to be in writing.
Whether this is tax return fraud and you owe FinCEN a SAR is a completely separate issue. FIN 2012-A005
So far, it does not sound like it.