You have to turn off system beep in the ALSA mixer. It's part of ALSA.
But why would you want to do that? The system beep is like... man it just brings back fond memories of my old 286 with 4mb or ram run DOS 5 back in the day. I remember, I'd put a big floppy in and that that system beep would just ring through the house, man. Good times.
System beep rocks.