There are 2 ways to disable the on board video card:
Step 1: Click Start, Control Panel, System, Hardware, and Device Manager locate the video card name under Display Adapters select the video card and then click on the small computer with the purple x to uninstall it from your computer.
Step 2: Disable the video card in the BIOS, if there is an option but most low end motherboards which are the ones who has integrated video cards does not include a video disabling option in the BIOS.