Hi Alex,
It sounds like you may be giving a musician SQ-MixPad?
The app for musician monitor control is SQ4You, which only allows control of Auxes (not mains).
When a user is set to only have access to one mix, this is the only one they will be able to see in the SQ4You Assign Mix screen – so there is no possibility for them to control the Main LR processing.
This is also the reason you can block access from the MixPad app (just in case someone tried to log on with MixPad using their SQ4You login).
Hope this helps,
Keith.