That's right, many people abuse this because once they relax and allow the bot to play, they know that the bot will start to randomly attack their friends and enemies alike and there will be no retaliation, because nobody fights robots until all other human players are defeated or weakened. Then the smart ass "wakes up" in an opportune moment and beats his startled opponents.
Definitely do not allow anybody to re-join after second missed turn.
I constantly check status of "inactive" players. I started doing that after humans suddenly "woke up" and attacked me unexpectedly. If I see that a status has changed to active, then I regroup my armies and increase defense. I strongly suspect that many players go inactive in order to be left alone when they are losing in hope that they can recover and attack someone later.