[AccessD] How to Cancel Selected Item in Optiongroup?

I am stuck on this one:

I have an option group with three radion buttons (value 1, 2 and 3), with
the  On_Open event of the Form setting the default value to 1.
Now, if a radio button that represents either value 2 or 3 is clicked the
client wants a message box to appear with a 'do you really want to etc....'
with Yes/No command buttons. If response is No, the event should be
cancelled.

I've been playing around with the beforeupdate and afterupdate event of the
option group.... can't get it to work.

Any suggestions?

Thanks
Borge