If you have no data in KNVV then you do not need to execute the SD part of FI_ACCRECV archiving.
If you have nothing in KNB1 then you do not need to execute the FI part of FI_ACCRECV archiving.
You need to remove the relation to the customer from your business partner. I have no Business Partner in use. But it should be in transaction BP after clicking the relation ship button.
Best you talk with your functional consultant if you don't know the BP transaction