I never have given access to my wifi, nor my Netro unit, to those who winterize my inground system.  They can generally do all they need from outside.  You do need to turn off your water supply to the sprinklers before they can clear the lines.

On the app, under Settings->Preferences->Rain, have you checked these settings?  These control which conditions, either actual, or predicted, rain amounts will change the watering schedule.

Settings->Zones->[Click one zone]

At the bottom of each Zone settings should be an "Enable" toggle.  Toggle it, so it is not enabled.  Save.

Repeat for all of the zones you wish to disable.

OR

If you want to disable all zones: Just unplug your Sprite.  It cannot trigger watering if it is unplugged.

Neither of these approaches should cause you to lose your default schedules.