As suggested, modify rule to require best competitive effort to win each game. True, this would do nothing to stop sandbaggers who have no problem lying. If this were the rule and after suspicious behavior you asked them if they were trying their hardest to win, they're look you in the eye and say yes and make up a story to explain away their intentional bad play.
But it would make a difference for sandbaggers who are honest and believe in following the rules. As evidenced by this thread, you can currently both sandbag and play by the rules. If I were a sandbagger, I would have no problem doing it because it is not prohibited. But, because I don't cheat and wouldn't lie in a competition, I would stop sandbagging if the rule were changed because not trying my best to win each game would no longer be allowed. I think a not-insignificant percentage of the player base would do the same, and so it seems to me to be a worthwhile change to make. It wouldn't completely solve the problem, but few solutions ever do, perfection is the enemy of good, and good is still, by definition, good.