No you can't turn it off, but why on Earth would you want to? The Niro's gas engine is designed to auto start-stop, it doesn't cause extra wear and tear like it would with a "normal" gas engine. It's what hybrids do. If a hybrid didn't do that, then there is no point in the car being a hybrid. Minimal ICE engine use is literally the entire point to owning a hybrid. If you don't want that function, then a hybrid is not the right car for you.
I agree that idle start-stop is annoying and unnecessary in a regular gas-only car. But it is an integral and critical function for hybrids.
If you hate that function, then a regular Subaru outback or something might be a better choice.
A few years back, I rode in my friend’s Mercedes Benz GLA(?) with idle stop/start and it was annoying. The engine kicking in when you stepped on the gas from a standstill was a bit jarring. I expected a luxury car to be smoother. I can tell when the ICE kicks in with the Niro, but the fact that the electric motor has already started the vehicle moving, it doesn’t bother me.
ISG in a hybrid is a completely different experience than in a regular ICE vehicle. We rented an Audi TDI in Italy that had ISG and it was a bit intrusive. It added a perceptible delay on takeoff and the noise and vibration of the starter was noticeable. Compare to the Niro, where the electric traction motor can handle both starting the engine and propelling the car from a stop. This means the process is essentially seamless and almost undetectable. If I pay attention, I can tell when the ICE starts but it's very subtle. There's no noise as from a traditional geared starter and the computer definitely increase motor torque to compensate for the load from the ICE starting up so the whole thing is very smooth.
In a hybrid, especially a PHEV, the ability to start and stop the ICE at will is absolutely essential so if it's a deal breaker, a hybrid is probably not the car for you.
If you haven't already, take a Niro for a spin. It's a pretty slick hybrid system all around.
It behaves nothing like start/stop cars which are at best annoying. Even if you try real hard you can't typically hear or feel anything. That's because there's a 60hp electric motor which is what is launching you vs in start/stop vehicles the gas engine is what is propelling you. The Niro will launch with the electric motor then (if necessary) the gas engine will kick in seemlessly. OR if you have it in sport mode the engine doesn't shut off which seems not efficient but actually works out really well. While at a stop the gas engine charges the electric engine so that the gas burned while at the stop is not just wasted. It is quite quick off the mark in sport mode.
I rent a lot of cars in Europe when I travel, and most have start/stop. Thankfully, most have a button to turn it off. It's extremely irritating in a gas-only car. You hit the gas pedal and nothing happens, then the engine turns over and you start moving finally. It's really obnoxious.
I think that is what the OP thinks the Niro does, too. My recommendation would be to take a test drive in one, you'll be pleasantly surprised by the instant throttle response from a stop. It's actually better than most gas-only cars, because electric motors have instant torque available. If you hit the gas pedals at the same exact time, my Niro is probably faster than my 500hp Audi with Quattro and a 5.2L V10, for the first 10 or 20 feet.
I think the instant torque availability from the electric motor is really underrated, you really have to do a test drive to see what I mean.