The way I have done it is to gray the box manually and disable all the bindings:

    $listbox->configure(-foreground => $grayout_color);

    # Disable all bindings.
    foreach my $seq ($listbox->bind()) {
       my $code = $listbox->bind($seq);
       # Store the $code somewhere...
       # if you want to re-enable later.

       $listbox->bind($seq, "");
    }
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