I beleive you will have to remap the rudder function on the joystick to something different. What you need to do to try this is go into setup/joystick/select joystick. The different axis are in there. The z axis on the joystick is the one you want to change. I am not sure if you can dissable it but at least try to map it to another function. I beleive the problem you are experiencing is the fact you have two z axis active at the same time.
If that doesn't work, you could dissasemble the joystick and unhook the rudder function. This would require caution and a modicum of knowledge of working with electronic devices. Whatever you do, unplug the joystick from the computer before trying anything like that.
