It should spin off when you stop the motor from turning. Use a long flat bladed screwdriver positioned against one of the teeth of the coupler on the angled side of the tooth. Then hit the screwdriver with a hammer to drive it into the plastic tooth without breaking off the tooth. (Note: it's a left-handed thread, not the more common right-handed thread. So you need to turn it clockwise to loosen.)
The hard part is getting that motor connection to stop turning with the coupling.
If you want to save the coupler and some damage to the blender when the screwdriver slips, get the blender coupler removal tool. See http://www.kitchenaidblog.com/blender/kitchenaid-blender-rubber-coupler-9704230-with-removal-tool/
for a link to buy both. Slide the removal tool under the old coupler to hold the knob from turning. Then turn the coupler to remove it. For some a pair of pliers helps you start the coupler moving. (It can get stiff over time.)
I hope this helps.