I didn't exactly disconnect my siren, but I did run the wire through a small toggle switch that I mounted in the left side of the case. Pretty easy to do if you're moderately handy with a soldering iron -- the wires going to the siren are easy to locate once you open up the case. Be careful, though -- the wires are of a very small gauge and the solder connections to the circuit board are easily broken.
Of course, if you just want to disconnect the siren, all you need is a wire cutter.