It's not hard, but actually I think you can open the config.xml and where the sens buttons are assigned you just put nothing in the field, so it becomes unassigned. Then you can set your sens using the numbers ingame and be done with it. Should work although I didn't test it.
This is what I'm talking about in the config.xml (located in your FGunZ folder), try this:
<MOUSESENSITIVITYDEC alt="-1">-1</MOUSESENSITIVITYDEC>
<MOUSESENSITIVITYINC alt="-1">-1</MOUSESENSITIVITYINC>
Edit: Worked. Closing this thread.