You mean if you click on the X Axis in the setup, and then choose the mouse and click the Set Axis button, it refuses to map to the mouse?
If you absolutely cant get it to stop choosing the joystick for some strange reason, then just unplug the joystick. Of course you'd have to restart the game each time you did that ( I think ), and then restart everything again when you want to use the joystick again. And of course that means recalibrating every time as well............
For all the trouble you are going to go through to use the mouse for gunning a buff, surely it would be less of a headache just to buy a stick you like better? What is your specific complaint with using the stick when you are gunning?