any npc approach is plain waste of time, even with tons of customization you can offer :P
- sending sysmsg, spamming chat, unique title, unique ID = easy to filter
- you might apply some abnormal effect for target, but it will be hardly visible
- there will be tons of poor english players who didn't understand warning and wonder why they are keep dying, and then quit
sending html/tutorial window is better because it's visible instantly and takes all focus of legit player,
captchas are good, because parsing htmls/ORCring require scripts and most of bot users are dumb.