Go Away.
The app for when you want people to go away.
(also customisable for just about any other message!)

Not only does it have 'Go Away' written in large clear letters, it will even say Go Away when you tap.

For particularly persistent annoyances, you can even set the app to randomly play the 'Go Away' sound every now and then to drive the message home.

If this doesn't make your message clear, then you may have to resort to post-it notes on peoples' heads.

All of this is completely customisable.
Pick your own title, and image, and even record your own message.