We had this problem with the dough blade. The plastic deforms and it starts slipping on the spindle, this heats it further so it slips more, and the whole lot welds itself together.
I used a mini drill with a cutting blade to hack the dough blade into pieces so it came off without damaging the motor spindle.
Bad design IMO - the flats on the spindle aren't deep enough.
Ian Oliver, August 2008