How to stop switchable graphics from switching to high-power GPU when charging the laptop?

Right-click on the Desktop and choose the option for "Configure switchable graphics". In the dialog that appears, you will see an option for "Automatically select power-saving GPU when on battery". This option controls whether the graphics switch automatically. If you uncheck the box, then the GPU should only switch when you manually change it using the specific buttons.

(Note, this is the procedure on my HP laptop with ATI switchable graphics. I assume it is the same or similar for your Acer, as the drivers should be the same, but I can't guarantee that it'll work.)