I did this for my IOpener webserver a long time ago.. it was very frustrating at first.
There are a bunch of IRQ's in the Right-Hand panel of the Power-Management setup-screen.. find the IRQ that corresponds to the Mouse, and set it to "Disabled".
You might also want to play with the settings related to Hard-Drive activity. In my IOpener webserver, I had to disable these as well -- otherwise, the backlight would turn on when Linux wrote its cache to the hard-disk.
I just retired this machine from web-service (upgraded to a nice, beefy P3 for JSP/Servlet hosting). I'll see if I have time to pop into my setup console and review the settings I used.