How can I deactivate on-board AGP adapter?

Hi, I have a motherboard from PCChips where the on-board AGP adapter is not working correctly anymore. I installed a PCI adapter which works fine. However XFree always tries to load the modules for both adapters, and crashes. How can I tell XFree not to use the AGP adapter? There is no possibility to deactivate it using jumpers on the Mainboard. Tx for the help. Jean-Gabriel Duquesnoy