Man, people who whine about metagaming without examining the deeper problems at play are the worst. These are basically three separate issues.

First of all, what's wrong with knowing AC? It's not like it's mega-privileged information, and it's almost always self-evidently derived from the creature's stats (either armor AC+dex or armor AC), so it shouldn't be hard to guess after a round of combat. It's information that, 99% of the time, should be intuitive. A dragon has high AC, a bandit in leather armor has low AC, a furry monster has medium AC. It's also not terribly valuable information. Half the time, players will just remember the number off-hand and then... their behavior will not meaningfully change? What's the issue here?

I (and a reasonable number of other people) consider it bad play to try to cheat and steal from party members without the party's out-of-character consent. It's essentially PvP. Maybe it's annoying to whine about it, but it seems to reflect a larger problem in the group. Talk to your players about their expectations in the game. If the thief is stealing from the party, it's OK to complain about it even if 'your character didn't know about it'. Maybe that player didn't sign up for a game where they distrust each other.

Reading ahead in a published adventure is cheating, not metagaming. It's just straight-up cheating, and at that point I feel like the correct thing to do is to confront the player about it and have a discussion about what your expectations are about the game you are going to play is. Ghosting your friend because you find their behavior to not be in line with the behavior you want is actually a really shitty thing to do and doesn't really solve much of anything.

These are distinct and real problems with distinct and real solutions. In some cases, it's a desire to interact with the complex elements of the tactical simulation of D&D. In some cases, it's opposition to PvP-like behaviors. In some cases, it's a desire to 'win dungeons and dragons', which I would argue is the real problem that makes you upset. The solution isn't to label certain players 'metagamers' and lambast them for their sins. Talk about your expectations. Decide what kind of game you're going to be running and talk with your players to make sure that they're interested in playing that game and willing to do so.