You don't need to kick afk players. All you need to do is add something that after 24 hours gives you 90% exp penalty and unable to lot on drops. When you can't do anything without logging out, THEN you'll have people do it on their own will.

And if anyone wants to argue that it will ruin the game for people who play 48 hours in a row, then feel free because I don't care about that minority.