This is simple: Just turn off your chat. So first of all, you know how there's a list of people that you can chat with on the right end of your screen? I will refer to that as your chat list. So first, go to options, which is a picture of a gear that is located at the very bottom of your chat list. You can then select Turn Off Chat to turn off chat for everyone or you can go to Advance Settings so you can select which friends/group of friends can see you online and which can't. Then the next time you get on, the chat settings will be off entirely from everyone or for only certain people.