It seems as if there's a few mods out there that increase legion assassin spawn rates as some people have issues never seeing them. I on the other hand have never had this problem and after having killed them hundreds of times I've gotten a bit sick of it. I suppose you could just disable them completely but I think it would be better to somehow have a quest that leads to them no longer attacking you. Seeing how I have killed every single legion troop in the game other then the respawning ones it seems like they should really just give up.