Promising not to call out individuals is not the same as promising not to call out cheating. A generic statement like “we know some people are cheating, which detracts from those who are logging big numbers legitimately” calls out the cheating, while acknowledging that big numbers are not necessarily a sign of cheating (especially since some people might be cheating by reading nothing and logging 60 minutes a day). So it may be possible to confront the problem without repeating the ugliness of last year.
Two questions for you, though:
1. Where are you in the read-a-thon (i.e., how long has it been going on and how much time is left)?
2. How much trouble would it be to change some of the rules after starting?
If it’s late in the game, I don’t think there’s anything that can be done. Even if the rules can be changed on a whim, there would be a lot of upset to do so right near the end (especially among those who are playing fair, which is the group you want to protect). The only thing to do would be to focus on fixing things for next year.
But if it’s still very early on and the committee can change the rules pretty easily, they might consider resetting everyone’s minutes to zero and/or changing how they log minutes going forward (e.g., they have to record themselves reading aloud or listening to an audiobook). It’s not ideal given how some people read or listen (on the bus, while doing chores, etc.), and it’s still open to abuse (if someone uploads 100 videos, who is going to check that none of them are duplicates?), but it could help.
If the rules are set in stone, including how prizes are awarded, then maybe something can be added for those who are really doing the reading: an opportunity for them to talk about the books they’ve read (which demonstrates they’ve done so), and a new reward for doing so (e.g., a personal or small group meeting with the guest author). This requires the author to agree, of course, but it adds on a reward for people playing fair without taking anything away from those gaming the system. If necessary, this could be presented as an almost entirely separate activity (so that it doesn’t look like a rules change). It also allows the committee to do something without calling out cheating at all (if they don’t even want to do so generically).
Also, let’s take a moment to look at the bright side: these kids think that an author visit is worth cheating for!