You are never going to eliminate sandbagging with this current format, as many have stated.
For me a big problem this year is that it was simply everywhere. The pros - like mr. El Ron Hubbard - know how to do this subtly. The newbies and scrubs were simply out of control, One armed bandits, stupid beef conversations in earshot at almost every bank; at times it was kind of ridiculous. And you know how impressionable pinball people are - look at how many players think there's only one way to play any particular machine. As the line for the beef buffet got longer and longer, the portions simply became larger and the Grade-D steaks simply got tougher.
I think a statement before the later rounds would be appropriate - play your best, visible displays of intentional poor play are punishable. You can do this without giving people the idea to beef out, or without indicating WHY some people might be munching on Arby's on purpose. The fact that so many people were so blatant about this shows that most of them really don't know they are doing something that's against the rules or would be frowned upon.
I think this would be more effective, as I just can't see ratting out players - many of whom might be our friends - is gonna catch on.