Bit of a long shot, have you tried swapping the box to a different HDMI port on your TV or maybe use a different HDMI cable to see if that helps.
Which NowTV box are you using the standard black box or the smart box with the inbuilt Freeview TV tuner ?
Also, was the box working previously without automatically switching on or has it always happened since you bought the box ?
Could you also confirm the make and model of your TV ?
Should have also mentioned, that all the "1 - touch play" feature does on my smart box is if i press any button on the NowTV remote controller my TV (Panasonic) will automatically switch the HDMI source to the correct input on my smart box.
Where disabling the "1 - touch" feature on the nowtv box, i have to manually switch HDMI input source on my TV remote controller.
Switching my TV on, does not automatically switch on my smart nowtv box in my setup, even if the 1 - touch feature is either enabled or disabled.
Wonder if your TV remote controller is conflicting with the IR on your nowtv box ?
What happens if you switch on your TV with the physical on/ off button on your TV set or if you temporary (for test purposes only) cover up or hide your nowtv box from the line of sight of your TV remote controller IR command.
Does the nowtv box still automatically switch on when switching on your TV ?