Already described technique could use.
Code searches table for a 'blank' record and that record is used for the new data.
Have "Delete" button on form with code that sets field(s) to Null. Will also need code that disables the ribbon delete and keyboard delete. I disable the ribbon completely. Example of code to capture and cancel delete event:
Code:
Private Sub Form_BeforeDelConfirm(Cancel As Integer, Response As Integer)
'suppress default Delete Confirm dialog box.
Response = acDataErrContinue
'cancel the automatic delete operation
Cancel = True
MsgBox "Must click Remove Test button to delete test from sample." & vbCrLf & _
"If sample is in closed accounting period, Delete/Remove test not permitted.", , "Delete"
End Sub