madman, not sure what you are planning to do (replace all with banana posts) but I would verify the right - (currently a screw) threads are not bad. These should be left hand threads.
if good, I'd use channel C parts and throw away the screw.
What you want to do is remove the plastic thumb screws with the threaded screw. Remember, these are left hand threads. Once removed, using a Philips screwdriver, loosen the plastic from the screws (don't need to unscrew all the way; just enough to loosen). I had to use pliers on the plastic.
screw in the thumb screws (remember left hand) until tight. now you should be able to tighten the speaker wire with the plastic knob.
It took me a while to fully understand how these work and got mine working (see the
4 screw pic Al posted above and the thumb screws in my
link above).