Click on the "Home" card and select "Away" instead of "Auto". This will force the Tado to be in an away mode at all times, and keeps the temperature to a minimum you've set for Away (Smart Schedule -> Away).
While using the Away mode could be handy in some situation that is not the same as "turn it off"...
I second this idea and I am surprised/frustrated that it is not present in the application.
The only way I have found is to use IFTTT app to turn heating off completely, and still leave the hot water on once a day for a hour or so
You can use the following instructions to manually change the heating thermostat to 5 degrees until you cancel