Just kill them all even the prisoner.. I am a legate in skyrim and killed ulfric my self.. I am also finding a mod or a script for that but nothing. but it is possible. because i found a mod that all guards and imperial soldiers in the major cites and much more in Camps are calling me legate even i don't speak to, except for the imperial soldiers that escorting a rebel. If you found the mod for that please tell me I had enough killing, but I am enjoying it somehow. sorry for the bad English.