I don't disable delete key but handle this with form BeforeDeleteConfirm event. Users get educated and eventually won't use Delete key (or ctrl+alt+del) for this situation, if they ever did either. Doubt my users would.
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