I'm making most of this up...
My combos have an IsDroppedDown property. It is set using the following:
PROCEDURE Click()
This.IsDroppedDown = .F.

PROCEDURE DropDown
This.IsDroppedDown = .T.

PROCEDURE KeyPress
LPARAMETERS nKeyCode, nShiftAltCtrl
WITH This
IF m.nKeyCode = ENTERKEY OR (m.nKeyCode = ALTUPARROWKEY AND m.nShiftAltCtrl = ALTKEY)
.IsDroppedDown = .F.
ENDIF
ENDWITH

You can add a IsDroppedDown_Assign() with the normal code in it.

* In a specific combo that you wish to behave this specific way you could override the assgn:
LPARAMETERS NewValue
IF NOT This.IsDroppedDown = m.NewValue
This.IsDroppedDown = m.NewValue
This.Parent.cnt4Area.Enabled = NOT m.NewValue
ENDIF

Add a container, cnt4Area, to your form and put any controls that are in the shadow of the combo's dropdown area into it, but not the combo.

So the DropDown sets the container's Enabled property to .F. The behavior of the container is to disable the controls so any extra click is to no effect. The controls won't react if the container in which they live is disabled. Once the dropdown is over, the container is enabled.