Are you looking for a filter, or a prefix? With a prefix, I believe you could use the tiny checkbox inder the existing filter box at the top of the thread list, to eliminate any prefix you want not to view.
Of course for that to work, the Clones would all have to agree to re-prefix unlabeled threads, as BS threads are presently re-classsified when they land on the wrong side of the line. Also, it seems like it would be kinda hard to decide exactly which threads should be so treated, either via filter or prefix.

~S~