If you need to remove someone from your Google Talk Friends list, just right-click his or her name and select Remove.
When you remove names from your Friends list, you’re also given the option to move the person to your Blocked list. If you’d like to block someone from talking to you, check the box next to Block this person in the dialogue box that appears, and click OK.
If you’re a Gmail user, new contacts in Google Talk will also be added to your Gmail Contacts list, and vice-versa. Keep in mind that removing or blocking someone from your Google Talk Friends list does not affect your Gmail Contacts list.[source]