Anyone heard of a ICE kill switch?

Yes. You can shut off the injectors. Do it hard wired with a relay or with a wireless controller.

 

Just my 2 cents, I'd get the Toyota block heater instead. This is a kosher mod, and using it for a couple of hours will get your car warmed up faster (and shutting off), within a block or two.

 

i know bill wilson will pull over and shut the car off. then the ice won't fire again until you need it.

 

I think in the Prius this is very likely to throw error codes with such a mod. The best thing to do for these situations is to get an EV button installed, and switch the car to EV for coasting below 34mph in S2 and S3. Except for when the Prius is in S2 during warmup and S3 below 34mph, it does a fantastic job of only keeping the engine on when needed. I just wish you didn't need 5 seconds of idling in S3 to get to S4 mode (the hypermiler in me keeps the car in nearly constant motion, having to fully stop at a light means I didn't time it right is conventional hypermiler thinking!). The other mod that can be done is a temperature sensor mod to fake the coolant water temperature to allow the Prius to get into S4 mode more quickly and be able to auto-stop the engine sooner in the warmup cycle.

Here's a reference for the Prius Warm Up Stages for those not yet familiar and in reference to my comments: Prius Stages of Operation & Operating Mode Availability -A Reference Aid | PriusChat

If you had both a EV button and a temperature sensor mod, you should be able to basically achieve the same results as a forced stop of the engine.