If your breaking the spirit of the game, Sure I think they should be perma banned, along with AFK guilds (guilds where they just loggin to population lock another faction so their mains faction can just stomp over em) Siege AP exploiters, defense tick exploiters that multi faction guilds use to control who gets emperor, etc.
But this isn't something ZOS is particularly well equipped to handle, nor do they seem to feel it's their obligation to do so. It's just a fact of life. Toxic people, make the game toxic. there is nothing we can do about it.
All's fair in love and war. If you haven't previously heard of spies being used in warfare then you might want to get out more.