In general we hope that people have a positive experience on Facebook and want to communicate. But we know there are situations in which people want to stop the communication, so we have a number of mechanisms in place to address that situation.
The first is the ability to unfriend somebody, which essentially is when you've engaged in a relationship with them on Facebook and you decide you want to terminate that relationship. Either party in the relationship can stop a friendship, and there are a number of different ways to do it.
The easiest way to do it is to go to their page, to their timeline, and there will be a button that will allow you to remove that friend relationship. That will still allow that person to see you on Facebook. They'll still see things you post publicly, or in groups or things like that, but they won't see things you share specifically with friends.
If you want to go a step further because there's somebody who is concerning you, you can block that person, which is a stronger mechanism. That prevents that person, for example, from creating a message to you. If they've been sending you private messages that you feel are inappropriate, you can prevent them from creating messages to you by using that block functionality.
There are other situations that may come up. When it goes beyond simple contact that you may find objectionable, we also want to know about it and to take steps where appropriate. On our “help center” page, there's a button at the top right at the corner that says “report an issue”. That gives you information on how to contact Facebook when you have this kind of problem, and other problems with content that you see on Facebook as well.