...
Unplug it from the mainboard or clip the wires if your feeling adventurous.
If the sound is coming out of your speakers while you're running Windows:
Open the mixer (double-click the speaker icon), open the Properties dialog (File -> Properties), and find the "PC Speaker" entry, check it, click OK, and click Mute underneath the PC Speaker entry.
That should cover every possible scenario.
Cheers,