Someone recently told me "there's nothing you can do about cheating in larp". I don't agree with this. I think cheating can reduced in two ways: rules and culture.
Let's take blow-counting as an example. There were some players at Mordavia that both crew members and other players repeatedly complained about as not taking their blows, across multiple events and characters. People who would be struck by a veritable hail of blows or poisonous tentacles and not go down. On confrontation, typical excuses were "I still had hit points left", "a lot of the blows hit my cloak", "we don't count little flicky hits, do we?" etc, or just a sullen silence.
In such situations there is a real difficulty for the organisers. Should they give the player a warning and then stop them playing if the cheating continues? The problem with taking such action is that it can disrupt gameplay. Not only may the cheating player be upset, but they may spread discontent among their friends and bring unwanted out of character conflict into the event. However, if you let such cheating continue then many people may feel that the event is unfair and that the organisers are not doing enough to make people play by the rules. Deciding which option will cause the least damage to the larp is difficult.
While diplomatic approaches to cheating players can have some success, that is really the ambulance at the bottom of the cliff. The better solution is prevention. Prevent cheating being an option in the first place.
First, how can rules reduce cheating? The simpler your rules are, and the fewer hidden elements they contain (such as Hit Points, which are only stored in the players' heads) the less possible it is for players to cheat without it being immediately and undeniably obvious. If every strike takes a character down immediately, the only way of cheating is to say "it didn't hit me". It is usually very clear whether this is true, and only the most hardened cheats would pull this excuse out repeatedly. Similarly if all blows that land on armour don't do any damage, then there is never an unknown factor over whether the armour is providing protection. Except in the dark, it should be fairly clear whether an armoured or unarmoured spot has been struck. The cheater cannot claim to still have health or armour points left if there is no such thing, and it is clear if they should go down immediately and do not.
In terms of culture, I think what's needed is something similar to the SCA culture where people who don't take their blows are looked down on. The culture needs to be one that recognises that "winning" isn't the ultimate goal, what matters is enjoying the activity and helping create an environment where everyone can enjoy it. This is especially true in larps whether player character are likely to come into conflict with each other. There needs to be a very clear "spirit of play" in such cases that says that while your characters may be in deadly conflict, out of game you're creating an enjoyable experience together and won't resort to cheating because it would lessen the overall experience for everyone.