That doesn't "lock" the aux, it is always possible to change a source on a bus if you push the button on the panel.
It can be made more difficult either as I suggested above or by using an Aux bus that isn't mapped as a User button so that it has to be more deliberate for the operator to change it. I'll often use the highest number Aux bus I have & a CC that also has to be "manually" selected.
#Carbonite