I have a scene where a large number of redundant cycles materials have accumulated. I'd like to have them removed so that there's nothing left remaining in the materials slots at all.

I've tried all the methods I could find of deleting them so that they don't show up in the slots like this but none have worked so far.
That is selecting all objects w an active object as well and 'copy material to others', saving, closing re-opening, no luck. And the Ctrl L method as well.
Thanks in advance