Tell them that perhaps their lack of good sportsmanship cursed them
At the risk of sounding insane, I think this somehow works in tennis. I've seen cheaters go on to hit net cords that roll serveral feet on the net and then...fall on their own side. The luck just doesn't seem to be with them. A sudden wind will push their lob out, things like that happen. I get the feeling that they've angered the tennis gods, and they'll be cursed until they start to play more honestly. It's spooky.