I don't know about ammonia, but GardensAlive Pyola and the neem products with azadirachtin listed as the active ingredient were found to be effective repellents in a scientific study. You have to reapply almost daily as the petals open. It might be better just to cut all the open flowers and bring them inside, so the scent doesn't attract more beetles.

Try Windex With Ammonia. It has the right concentration of ammonia to kill them on contact but won't hurt the roses.