this happens to me in the school hallways, i usually just turn around say "come on man" unless it somebody who is really trying to be rude then i just stare at them. haha. Meh, i dont really care about it, its hair.
I try to keep this rule as well. If a stranger asks what it feels like I'll let them touch it, but randomers being wankers and pulling my hair? Not on.
Friends have learnt not to because it really pisses me off when I've spent time doing my hair and someone just comes up and ruins it for no good reason.
People are idiots and think that i cant feel it when they touch my hair. If its a friend its alright, if its a stranger i usually just turn around and stare at them. Its funny because they get all awkward because they thought you didnt notice.