How to make Tkinter Listbox entry insensitive?

[Skip Montanaro]

I have a pair of Listboxes, each with ranges of the same text.  For example:

ABC    ABC
DEF    DEF
GHI    GHI
JKL    JKL

(My apologies, I'm sure Gmail is going to butcher that.)

If I select "ABC" from the first list, only the last three values in
the second list are appropriate selections. Is there a way to make a
list entry insensitive? No amount of Googling found anything. Does Tk
perhaps use a word other than "insensitive" to describe an item which
you'd normally be able to click?

Thx,

Skip