[AccessD] A2K: Populate List Box

Hello all
This should be simple. But it is p***ing me right off.
I have an Events form called frmEvents. Bound to tblEvents.
2 list boxes on this form. The one on the left lists available clients
The list box on the right is showing people who are registerd into this event. (Attendees)
This is stored in tblRegistration. AttendeeID with EventID and RegistrationID.
(All the names of attendees are got from the tblAttendees table)
 - Pretty basic sofar.
Double click on left box to 'Register in to the current Event'
Double click on Right box to 'un regsiter' - Rocket science.
The lists grow and shrink accordingly.

But here is the problemn
In the list on the left I only want to show people who are not registerd into the current event.

I am having a 101 moment here (Drew)

Any help??

Darren