I'd think they'd fall more under "justice league" or "executioners" rather than "Guards" if all they do is kill... [Yes I know you suggest using arrest in the long run, but until that time] some questions the idea raises are as follows;
how that would fit with the game settings saying killing for a crime is rare?.. of course you currently die for spamming.. and they also say klyros can see better in the dark, and about a billion other things that don't apply, so who really cares what's implemented in relation to what settings say?... Or as someone said how death is hardly a meaningful punishment currently, affecting some races more than others depending where they die... and should people escape justice if they just died of their own fault, by the time their offence was reported/noticed?--it takes 2 to tango & when a pair duel in front of harns [first of BOTH are accepring risk of death already, second] if 'Guard" arrives after one dies & kills the other all it means is in certain areas /challenge should do the same as /die for both parties as soon as either accepts... unless we imagine the challege happening in town then they walk out to fight as i have known many to do....-- And what point would there be if they got the privelledge to fight without challenge? so that every fight in plaza means another fight in plaza?--and unless they got other major advantages how could they ensure winning? especially with more than one wrongdoer?--and you just know that upon seeing a guard just kill someone in the streets it could easily invite many people to try & persuade the guard to try picking on somene their own size etc...
My suggestion, assuming you want to stay with this idea of a guild of players hand-picked by GM(s), with some added power(s), to 'guard' or enforce law, is to skip ahead to the arrest part.--i would suggest allowing them a command that does 5 things at once: 1)--mute, 2)--immobilize, 3)--report, 4)-- opens petition window for [guard] to make their own report of actions etc that report log may not cover...5)--ediit description so the bottom says "You evaluate that this person has been incapacitated (or bound & gagged) by [guards], you see their crest on his/her bindings.
You evaluate that these bindings would be impossible to circumvent"
..The freeze & mute would stay in effect until a gm [presumably the one overseeing this group] arrived & decided to let em go, or whatnot... This would make sense for a group hand-picked for dealing with ic & ooc offenders...
BUT the real question that should come before any of that is: are you discussing guards for IC things or OOC things? or both? After all while i feel the people dueling by harn should have guards doing something about them, i feel the same about people rping as thieves & that those same guards should be able to take action against them... however i don't know that i feel someone spamming or talking out of character should really have to answer to those guards, as their roles are suppose to pertain to IC & for them to be 'policing' people's OOC actions makes their response equally ooc... and any ooc offences in current system are meant to just be taken up with gms... Like what was just said about /reports, it's already up to everyone to be vigil of such matters & everyone is already able to make equally serious reports...
Assuming it's more an IC thing, it sounds more like the sort of thing i would strongly advise be put as, if anything, an addendum to UtM's suggestion of the sort of dungeon--perhaps people could do good guy/law enforcement quests, then after say 3 people [greater/lower number depending on faction] with all/most of those "good" quests completed, use an "encarcerate" glyph [that can only be attained by certain quests & requires certain factions to use], on the same person, that person is teleported to this jail realm place... If that gets abused who cares? police can be corrupt, just makes 'evil' characters more humanized etc right?... And since the power granted is related to playerskill at questing, mechanic related, etc you can jail either for breaking the law or just "what you feel is wrong", andno one can whine about unfair jailing... The only kind of 'unfair' jailing in such a situation is a bit of a catch 22-- "I insulted him ooc so he jailed me"-- is just reporting yourself for breaking rules. if the person jailing you wants they could have decided to rp as "you looked suspicious so i jailed you without warning. deal with it."..