The small chat window in Gmail shows the status of all your contacts, with whom you can chat. To add anyone to that list requires one to send an invitation to the other person.
However, if you email other Gmail users frequently, Gmail has a default habit of automatically adding them to that list.
This can sometimes clutter your chat box. Here\’s how to stop that -

1. In Gmail\’s Inbox, click the Settings link on the top.
2. Click the Chat tab. Select the Only allow people that I\’ve explicitly approved to chat with me and see when I\’m online option.

3. Click Save Changes.

That\’s all!

More:
Add Google Talk gadget to Orkut
Report Suspect Phishing mail in Gmail and Yahoo

Merge Gmail, Yahoo, Hotmail, AOL, Facebook, Myspace accounts