I always try to find the noisemaking device first and try to kill it, but I seem to recall pulling apart a connector located right at the ignition key switch...I believe I got the info from a long, somewhat argumentative forum thread here. Snap the plastic cover away from the switch area and it's right there.
The nice thing is that I disabled all of the annoyance buzzers/chimes EXCEPT the one that I like to have: The "you left the headlights on, stupid" one.