I thought that baning one charracter just because its player has other charracter in enemy realm is against the rule.
This is similar situation - to ban one charracter just because its player has other charracter that is exiled from the realm.
Both situations are sending the same message: "Your charracter is unvelcome in the realm just because your other charracter is enemy of the realm."
If it is legal to act that way, then I have nothing to add and you are free to close this case.
