true sandbaggers do not care about league money. They usually hide in mixed leagues that pay small prize funds in the first place.
sandbaggers are focused on tournaments.
The will bowl their mixed league where they intentionally average 20-40 pins under their true ability and then enter the hdcp tournaments and clean up making a couple thousand per tourney.
No league format you come up with will stop them because they simply bowl below their ability. I have been in leagues where known sandbaggers bowl. They leave a 10 pin, they shoot at the 7. They intentionally try to miss the head pin and leave the fence row (7 count) on every shot. See by doing that, they are still working on their game. Instead of making the 1-3 the pocket their strike pocket. They are making the 3-6 "their" pocket. If they hit the 3-6, they struck in their mind. And their scores are of course lower because they don't string strikes. It is practicing in league for score.
The only way to possibly end this is for USBC to begin tracking all tournament averages for sanctioned bowlers. Then each tournament can decide if they want to consider usage of that average.
If a bowler has a book average of 178, and a tournament average of 210, might raise a few flags.