So I have a question, apologies for being a douche in advance. But to me this is a newish horse owner, or an owner that has just had great horses previously.
So now he/she is being told to move the horse on, and whack it on the butt or stomach? Well what is going to happen when this disrespectful horse takes off? Maybe it will take off in a circle and pull the lead rope out from her(his) hands, maybe it will make a rope burn in the palm? But more likely the horse is going to take off in a straight line, and pull the lead rope out of the owners hands.
I don't doubt the advise that has been given here, but you need to do whatever you need to do and bring the herd in first then take the horse to a round pen or small working area to use the advise given.