In previous Call of Duty titles the mouse was driven in game. In Black Ops 2 it is driven by the hardware, which is controlled by Windows. By default mouse acceleration is turned on in Windows.
Open the control panel. In the search bar type in “cursor” and underneath the “mouse” header go to “change the mouse pointer display or speed”. In the window that pops up you can change the speed of the cursor, which actually also affects your sensitivity in game. Underneath the slider is an option called “Enhance pointer precision” which should basically say “mouse acceleration”. Turn off that checkbox and click OK. Now you have turned off mouse acceleration.