Hi,
AFAIK this is not possible, I don't remember ever seen such a behavior on a
listbox, what you can do is make them unselectable, you can intercept the
SelectedIndexChange event and check if the new selected index is to be
disabled then just do something about it:
1- go back to the previous selected one, you will need to keep track of this
in a int variable
2- select next/prev
Hope this help,