How do you handle a person who harasses you all the time?

What kind of harassing, dear.

Best rule of thumb:

You tell them to stop doing "Such and Such". If they don't, you say, "If you do that again, i will do this" (report to authorities, teacher, boss, etc.) If they continue, you do what you said you would.

Don't ever threaten to do something that you are not willing to follow through on.