How To Stop A Form From Closing If Some Essential Fields Are Not Populated

I tried putting the following code in the form unload procedure, but although it does generate the correct message, it doesnt stop the form from closing.

Private Sub Form_Unload(Cancel As Integer)
If [Starttime] > 0 And [Admin_time] = 0 Then
response = MsgBox("Please click on the stop button to stop the clock")
Exit Sub
End If
end sub

If the above condition holds, I want the user to click on the stop button before they close down the form. I'm guessing that the form is already commited to closing before the unload event? :o