If you try to uninstall onboard graphics in the Device Manager, it will only reinstall the next time you restart the PC. It is the graphics driver that you uninstall under Add/Remove Programs. You disable onboard graphics in the BIOS as kpo already said.
That said, some systems will automatically disable the onboard graphics as soon as a graphics card is installed. In at least some BIOSes you can enable or disable that automatic ability. Others you have to manually turn off.
You can often run both cards at the same time with different monitors. However, if you do not intend to use the onboard graphics, I recommend that you disable it if it isn't done automatically. As long as it is enabled, some system RAM will be reserved for it unnecessarily and it will be wasted.