Don't force, shank could be easily cracked.If not glued for a previous problem (sometime i've seen pipese with that problem)you could try to put the stem into a glass full of hot water till where briar meet vulcanite.Then slowly turn on right and on left.If stem does not move stop you.I can't imagine other way
[Next time, put that pipe in the freezer for a few hours. The wood will shrink, thats all you need to do most of the time.
Sorry about the pipe Seanz...]
That was my hint also. The plastic should shrink more than the wood. Heating the bit will cause it to expand and fit tighter so do not heat it. You could heat the wood and cool the bit. Then the shank would expand and the bit would shrink which is what you want to happen. 8O