This question has been asked and answered before (did you try searching first like a good forum participant?

) I just did, and came up with
this thread for you.
In any case, I've haven't succeeded in removing my knob either, but I honestly haven't tried all that hard yet. Apparently it does screw off, but some of them seem to be glued too. I heard somewhere that a big pair of vise-grips will do the trick, but wreck the stock knob in the process (which only matters if you actually want to keep it for later).
Hope this helps!