Others have gone through the workflow with you, so I will confine myself to just answering your original question.
You cannot, strictly speaking, disable an item in a ListBox or Combobox, but you can use the AfterUpdate, BeforeUpdate or OnClick events to ignore such a selection.
I have a few such cases, where I have such a control with stuff I don't want the user to select, such as:
** Fruits **
Orange
Apple
Pear
** Vegetables **
Carrot
Potato
Radish
** Berries **
Strawberry
Blueberry
Huckleberry
In the event code, I would check to see if the user has selected a line that has asterisks, and not permit the selection of such a line. You can handle this in many different ways - flash an error message, just ignore it, go to a a default value. In the BeforeUpdate event, you can even not let the user leave the control until he has selected something sensible, although you want to be careful about that - it can be very irritating. At the very least, you should let the user leave without making a selection of any sort.
You can modify the source for your control to put an asterisk before the selections that you would have wanted to disable, or you could simply leave such items out of the source, although that can also be irritating, since user will sometimes remember that they saw something and wonder where it went. That's why controls are generally disabled rather than hidden when you don't want the user to access them. Since you can't do that with the individual items in these list-driven controls, a visual indication that an item is off-limits might work just as well.
