The Listbox is a Windows control. I don't believe I have ever seen disabled items in a listbox. And I don't see any property or method for disabled items in Microsoft's documentation for the listbox:
http://msdn.microsoft.com/en-us/library/system.windows.controls.listbox.aspxThe simple solution would be to not add the items you want disabled to the list. If needed, you could create two lists, one with all the items and one with the disabled items. Or if you need several different lists, then either make them all beforehand or build them as needed in the program.
James