Want people to leave you alone? Tell them you are depressed works like a charm.

I'll tell them I've got Aids...

Or that you're a JW recruiter.

Ask them if they have a minute to talk about god.

Yep, most people don’t want to hear it but in all honesty if I could escape from this brain of mine I would too.

Caution: Although that is true, once you say that people tend to label you (antisocial, mental, etc.). While they may not say it to your face, their actions show it to be true (and you might even hear them whispering it behind your back).