I am experiencing this same issue. I tried deleting the code and the events (off the form) and did everything over and I still have the same problem. Using Access 2013.

Private Sub IsQuoteToggle_BeforeUpdate(Cancel As Integer)
    If IsPaid Then
        MsgBox ("You can't mark this as a quote because the invoice is already paid")
        Cancel = 1
    End If
End Sub


In Access 310 around the 8 minute mark you show how to put some VBA code that pops up a MsgBox and prevents users from changing the toggle button from an invoice to a quote if the order is paid.  Then similar validation with the IsPaid check box if the order is a quote.  In both cases the code seems to do its job but if I click on any other part of the form the MsgBox pops back up and does not let me out of the routine.  If I try to close the form eventually I get a message that states You can't save this record at this time.