In Bios there is a section where you can boot to discrete and disable the iGPU, depending on the motherboard the options are titled differently but essentially the same thing.
Normally your graphics output is set to auto, change it to discrete and your iGPU should be switched off, alternatively with Asrock a hidden bios option will appear which you can switch of the iGPU completely.
Source:
http://www.tomshardware.co.uk/answers/id-1708507/disabling-apu-graphics.html