When you see someone on a specific board catch up like that a lot, speed up. Go an extra half mile an hour or so. Same thing if they can't keep up, slow down. Adjust your speed so that the board wants to go the same speed as the boat with that rider.
Dennis gave me that advice a long time ago, and it works very well. It should be a VERY rare occurrence for someone to hit the board with the boat. I think it has only happened once to us.