If you can't find a software solution just flip the mouse over, the ball doesn't usually move when it's inverted.

Another thought, get a Logitech cordless mouse and move it out of range of the receiver (~6ft) to disable it.

Update:

Use your imagination here. You could invert the mouse and place it somewhere (that exists or that you create) so that the buttons can't be easily pressed. Velcro® is a wonderful way to improvise mounting solutions for small electronic devices :-)