The relevant law is Law 47.1,
41.7 Bowling of dangerous and unfair non-pitching deliveries
41.7.1 Any delivery, which passes or would have passed, without pitching, > aboveabove waist height of the striker standing upright at the popping crease, is > unfair. Whenever such a delivery is bowled, the umpire shall call and signal >NoNo ball.
This is part of the standard laws of cricket.
So as the umpire you will call and signal no ball for all deliveries that did or would have passed above waist height of the striker. So if the player comes down the wicket but you believe the ball would still have passed the striker above waist high it will be a no ball.