When I was SPL I ran into a similar issue. We also spent plenty of time at the meetings going over what was needed and what to leave behind. We had a shake down the week before our little trip but wouldn't you know, stuff migrated back into packs. The most extreme example is the kid whose mom packed his pack for him. She wasn't at the shake down meeting and just loaded it up with whatever her son might need to stay safe and comfortable. Lucky for him the loop we were hiking passed relatively near the vehicles about half way through and my brother and I carried two extra pair of clothes (consisting of jeans, t-shirts, socks, and underwear), an extra cotton sweater (we let him keep one), brand new roll of toilet paper, and a number of other items back to the cars. The two miles my brother and I walked with all of his stuff gave him time to recover.
As has been said, the best thing you can let them do is let them learn through experience. Now that they've experience what its like to carry a heavy load (and hopefully seen people having a more enjoyable time with lighter packs after following the advice that was given) they'll think twice when loading their packs for next time. Or maybe not let their mother pack the whole thing for them :P.