Add a section like this:
Option "Protocol" "IMPS/2"
Option "Device" "/dev/input/mouse0"
Of course, the protocol can be different, and check which device is your mouse, with:
where x is some number, move your mouse and see if there is garbage on the screen. ctrl-c to end that.
In your serverlayout, tell X to use the device:
InputDevice "Mouse2" "SendCoreEvents"
That should do it.