This suggestion could cause, that players thieves will need to steal you and LEAVE, or they will need to steal only something from you (i.e. 1/3 of caps), so as a player you would notbe sure and you will rather accept some loss in opposite of being killed by npc guards, if you were wrong.
Ok, look, you're civilian and I'm a thief.
You're walking on street and singing some song, I'm steal tagged thief-alt who can't do anything in his life except for stealing. You decided to stop to trade with... damn they removed Sha Enin after wipe, well trade with some unimportant trader. Then, I use my super-puper steal skill and possess all your items, no one noticed it, except for some strange guy, there, in the sky who controls your avatar. And then you, very very mad, demanding justice, punishing me. What will it be from guards point of view? Some crazy guy hitting innocent bluesuiter for no reason! Need to stop that mad guy and kill him. It's ok. Because no one noticed my stealing, even your character, but it doesn't matter, when you see that your items disappeared
after successful steal, it's the same as if your character discovered it, there's no way to prove that it's your items, because your character didn't warn guards in stealing process, so if you'll demand justice, it'll be against town's laws, as there's no proof, no proof - no crime. Of course there must be some ways to keep an eye on one person or something, or, as it was said already, make thieves risk something, because they're stealing in bluesuit anyway, and in case of fail/death they don't lose anything, but I really have no ideas how to make thieves risk something, except for reputation may be.
So here's my suggestion:
If your reputation is higher that reputation of thief, guards will trust you and you can always shoot thief if he stole anything from you, even if he haven't failed, also thief will lose reputation after that and lose protection (after your attack, if he stole anything from you). So thieves will need to always have high reputation to act like now.
Also there could be some message when they successfully stole from you, but has less reputation than you, or no message...
So thieves 1st: Need to earn reputation.
2nd: Be careful when steal from high rep people, because even if they succeed, they still can be killed and still lose rep.