I am not familiar with the way that the motor drives the spout on a snow blower, but I am picturing a roller chain sprocket or a ring gear around the outside of the spout. Could you do the speed reduction mechanically by changing the driving sprocket or gear size? If that is not possible, I can't think of a good cheap way to do it that would not reduce the torque so much to make it useless. A servo approach seems the most obvious but fails the cheap test. You might try putting something in series with the motor like an electric stove element or water heater element, but anything that heats has the undesired feature of increasing in resistance as the current requirement increases.
Mechanical limit switches on the spout and living with the fastest spout in town might be the cheapest option. The limit switches should control a relay in order to keep them a manageable (and cheap) size.
BobH from sunny Tucson