What type of motor and what type of sprocket? I assume it's a 7/8" shaft with a 3/16" keyway. Keys are not designed to fix a sprocket onto a shaft axially, only radially. For axial fixation we use the supplied set screws and occasionally shaft collars. It sounds like the sprocket was installed with an oversized machine key which makes removal difficult. For this reason I always use and recommend undersized keys (I'm talking a few thou here) which are available on Mcmaster. Another possibility I've seen is somebody putting a drop of 680 in a keyway. Before you torch it, can you post a picture or two? If it a matter of needed some "gentle" brute force, the correct tool to use is a large arbor press. This can be tricky depending on the size of your motor.