Add a “Main mix” lock option on user settings for MixPad use. I know that if the SQ4You app only allows a single mix to be allocated but the MixPad app to me more intuitive and therefore is easier to teach people to use.
It’s already possible to lock all the Aux mixes or have only one Aux mix available to a user so they can only adjust their own mix but there is no way to prevent them adjusting the main mix or its master level which seems nonsensical to me.