As long as the nut on top of the bar clamp is still tight, I don`t think smacking the plastic will do any good- the top nut is still holding the bolt, so the wedge can`t go anywhere. Need to loosen that top nut first. See if you can clean up the flats with a file enough to get a good wrench on (better a regular open end wrench than an adjustable). If not, you`ll have to use Vise Grips. Loosen that nut first, THEN try the wedge (hitting the plastic handle). The big, flat nut at the bottom of the stem is definitely the headset locknut- I wouldn`t mess with it until you`re able to get the stem out because all that will do with the stem in place is let the bearing rollers fall out.