If you tell me a good way how to prevent that kind of exploit, feel free to post a suggestion in the appropiate forum. We're thinking about the matter ourselves, because we consider it serious as well.
First of all, i don't think that people related to attackers AI fief should be prohibited from joining the defense thats just stupid. Instead i offer you system that would punish those who are trying to sabotage AI.
In my opinion there are two easy ways to encourage fair play:
1.
"Approved status" The idea is simple: using two existing features Reliability and Performance make people think about how they play the game.
For example, introduce "Approved" status for players who have summ of Reliability and Performance above 15 (or any close value that makes sense).
The only advantage of being "Approved" is getting full amount of experience, people who are not "Approved" get either reduced exp or no exp whatsoever.
Basically, if you don't slack and don't miss battles - you already have 10 points from full Reliability, getting 4+ Performance should be no problem unless you are wasting tickets, of course.
2.
"Leaver status ranks" Whenever you miss a battle you are signed up for you get "Leaver" status of a defined rank. If you miss other player battle - the battle owner is free to punish you (by giving leaver rank) or forgive you. When you hit Rank 1, you are unable to join strat battles for 24hours, Rank 2 - unable to join strat battles for 48 hours, Rank 3 - 72 hours etc...
After 10 consecutive games played without leaving, "Leaver" Rank is set to 0.