Grip the fan with your hand and turn the pulley anti-clockwise, though it's best not to add any lubricants as this will loosen the fan on the armature making it harder to dismantle...
If it is not turning at all, you may have to strip the motor down so you can grip the armature (with your hand preferably, I destroyed my G5's original armature using one of them jar opener things) and try again...
