Press the CUSTOM button on the left earcup until Ambient Sound is turned off
Use the Sony Headphones Connect app and set Ambient Sound Control to Noise Canceling
If Speak-to-Chat is enabled, turn it off in the app to prevent ambient sound from activating automatically
Make sure Adaptive Sound Control is disabled in the app if you want ambient sound to stay off
If needed, reset the headphones by powering them off and on again after changing the setting