I know how anti-NPC people are around here, and for good reason. However, if crime and punishment are to be balanced, it only makes sense that we're going to need guards at some point in time. They allow for someone to continuously protect their claim, without actually having to be online at the time. However, they also allow for strong players to take what they want, so long as they can beat or sneak past the guards at a certain place, which is a lot like real life crime.
However, since people are against NPC's, I thought up a different solution to the problem which wouldn't create characters with no depth needlessly. The solution is guard dogs. Obviously, this would have to wait for animal domestication, so it's probably a long ways off, but it allows you to create a guard without creating a non-player character.
As to the mechanics of the thing, here's how I think it would work. Dogs would patrol a certain area, be it a claim or something else marked out by the owner. If they come across a crime scent (which I propose should be made invisible to humans), they'll track down the perpetrator if he's within a certain distance, and then attack. Whether dogs would kill or simply KO might be something else for the owner of the dogs to decide. Perhaps every in-game day, a dog must go and rest at a kennel, or other similar building. This building would be a requirement for having dogs and/or training them. You could also set your dog's options here (ie, where to patrol, whether they should kill or KO, et ced.). Therefore, it would be optimal to have multiple dogs on multiple schedules.
You could also institute a tracking dog, which would work similar to the current ranging skill, only the dog would simply go after the scent so long as you were following within a certain distance. This would lead you straight to your target.