A rule about this is completely nuts. What someone does with their own book is nobody's business but theirs, and trying to police it in a club is just going to make people feel judged for no reason. Our club has the same mix, some people scribble all over the pages and some keep them spotless, and nobody cares because it has zero effect on the actual talk. If notes help you remember stuff, write them. If marked up pages bother you, use your sticky notes. Both work fine. The real issue here is that two people turned a personal habit into a fight, and the club is about to reward that by making an official policy. Let it go and get back to talking about the books.