Normally, motherboard BIOS will detect VGA card and disable on-board VGA automatically.
However, you can still manually disable it in BIOS.
1. Press Delete key in POST screen to go to CMOS
2. Go to Chipset> Internal Graphics Ocnnfiguration> Internal Graphics Mode> Disable
3. Save and exit the CMOS setting
It will disable on-board graphic for the motherboard after restart.