I saw many reported Bugs of this Problem here, but its almost 1 Year ago since last activity of any Dev's.
Its still not Possible to Lock your Mouse to 1 Screen while playing Arma in Fullscreen!!
The Mouse is still moving out of the Screen and is hitting other Progrmms or something else opened on 2nd / 3rd screen..
It would be awesome.. REALLY AWESOME... to get a Fix for that after a Year...