Model 17 will not stop unless unplugged

Your switch that switches that outlet its plugged into is bad and needs replaced. These will ALL run as long as they have power to them at the plug.

This model has a batch feed switch, right? I have never installed one with a plug on it, tell us more about how this is installed and wired. http://www.insinkerator.com/pdf/H532.pdf

Ballengerb is correct, this is a batch feed that turns on and off with the contacts on the disposal stopper, it will not run without the turning the stopper in the top of the disposal. I have seen them wired with a plug into a wall outlet but never working off a wall switch. I would think switch in the disposal top is either stuck in the on position or broke in the on position. The disposal should not run without making contact with the stopper when it is turned to start the disposal. Try calling insinkerator service at 1-800-558-5700 they may be able to give you a answer to your question.

In addition to the above advices: look under the sink. There is a Crescent like shaped attachment on the collar of the disposer. It is hand-snapped onto the collar. See if you can un-snapped it and snap it back onto the collar.