if you know of someone up there get a beretta, an ak or a g36, TAP every shot, crouch on top of the FIRST crate on your spawns exit, you should be using a dot as a crosshair, first WATCH him/her get close to the edge, he/she will shoot and the retreat, take notice of that mark since its the most outward he/she will move, keep yer crosshair at the same level (altho be ready to move it horizontally) and shoot, 1 or 2 shots with an AK and he/she will think it twice before going there again.
you can also use a grenade launcher, just be sure to set it for big distances shooting and aim several meters BEFORE the edge of the platform but keep in on the middle, thats so funny coz he/she will either jump or die lol