In most cases you can increase your salt level to the upper limits of your system, which will help extend the operation of your salt system in cooler water temperatures.
However, whether you turn the unit to 0 or turn the power off, it should not energize the cell. Therefore, you will have to resort to adding chlorine by hand until it gets warm enough to reactivate your salt system.

Another option, add a heat pump and keep it heated! = )