If your flash has slave function, set it up on the opposite side and point it directly at the offender and manual zoom to maximum. Every time his flash fires your flash will also fire right at him and cause a big flare in every shot.
Actually just stand upstairs on the second floor will do. Bring 2 flashes or more..... and point directly at them. It will disturb the rest of the photographers (which is a little unfair) but who cares. This will make them upset and push those ladder gang down.......SUPER EVIL THOUGHTS.......WAHAHAHA