To sum up, if you're in the BIOS, you can disable it somewhere there, but unless we get a lot more information on your BIOS, we can't tell you where. The BIOS menu generally isn't incredibly complex, though.
If you can't disable it (which is unlikely) move it down the boot order so it's the very last item. If it still gets to that point, you have bigger problems than not being able to disable PXE or MBA booting, as it means you have no bootable devices whatsoever.