Ausad, I've just made sure that any user I do NOT want to use the Manger Control window has security access LESS than "Manager" level, i.e., "Cashier" or "Employee" or "Lowest".
If you currently have someone with status of "Manager" (as set in the Employee window), and you don't want them to mess around with how you have the Function buttons set up, I think you just need to move their status to anything below "Manager", rather than changing anything in the Security Control settings.