Cut all flowered shoots out completely. Also trace old wood....lighter more gnarled wood...as far back as possible, to ground level if poss. Mock orange does look " silly" when cut back but it soon recovers and produces shoots to flower next year. I wouldn't cut bush back totally as you will end up with a plant that prob won't flower for 2 or 3 years. Don't cut back to "thicken" it up, please. Prune immediately after flowering. A potash feed too may help