What I do is, if you don't want to talk to ANYBODY. You can just go to Chat, Options, Go Offline.
However, if you want to appear offline to specific people, or only want to talk to specific people, what I do is, I make a separate friends list, which you can do on the Friends List option, and just Drag and Drop the people you want to be online to, and just click the little green switch next to the Other Friends List so that you appear offline to them. :]
Hopefully this isn't too confusing.