You can't kick AFK players out. A few members of my FC and I were doing Praetorium last night for story purposes and one of the tanks AFKed from the start. So the rest of us left rather than try to defeat all of the bosses in that dungeon with just one tank. And it's not like Praetorium is even a good dungeon for farming tomes. I think in a situation like that I'd rather take the 15 minute penalty than waste a lot more time only to fail at the end.