When you have the dash unit set to A/C at it's coldest setting , there may still be some coolant flow through the heater core , warming the dash .
Switching to Max A/C ( recirculate ) should shut off coolant flow through the core completely .
You may have an issue with the amount of refrigerant in the A/C system , but the operation of the coolant flow control valve needs to be checked too.
Many owners have installed a manual shut off in the heater hose to aide the A/C system operation.