There is a button on the dash that turns it off however it resets back to on the next time you start the car and you have to remember to turn it off again.
We used to leave out ours on the in the summer as it worked perfectly and turn it off on trips in the winter months due to the excessive loads on the battery. Also it can be a pain in slow moving traffic so I used to switch it off then too.
I believe you can take in to a fiat dealer and they can load a program upgrade that leaves it in the last set position when you re-start the car, so if it was turned off when you stopped it would stay off when you next come to use the car. It was something I was going to ask our local dealer about but we've traded in for a 500L now with no start stop.