I'm not sure you can restrict a player from moving, and allow him to shoot at same time.
I think you will need to script these restrictions, and make a plausible explanation for these restrictions so you don't piss off the player. You can add scripts and/or triggers that will detect when player is X meters away from your desired fixed position. When detected, you can have his buddy or leader radio him "You will be spotted if you leave your position. Return to position immediately!" If player continues leaving position, you then detect that, and have the enemy shoot him, or a minefield blow him up, or his own officer shoot him for disobeying orders...etc.