Blocking a contact prevents the person from talking to you and seeing when you're signed in to Google's network. You can block and unblock a contact with these instructions:
Blocking a contact:
- Type the name of the contact you wish the block into the Search box at the top of the chat list.
- In the box that appears, click Chat to open a chat window with that user. They will not be notified unless you type a message into that window.
- In the chat window click Actions menu.
- From the menu, select the option to Block the specific contact.
Blocking a contact will block them across all of Google's network.
Note for Google+ Users
- Blocking someone from within the Chat list or a Chat mole does not also block the person in Google+. So, although they won't be able to chat with you, they'll still be in your Circles and be able to see your content. However, if you block the person in Google+, in addition to being removed from your circles and extended circles, they will also be unable to chat with you in any Google Chat property (i.e. Gmail, orkut, iGoogle).
- Google+ synchronizes all of your blocked contacts across all Google properties. If you have contacts blocked in any Google products such as Google Reader or Google Buzz, these contacts will now show up as blocked in your Chat list as well. Unblocking them from within the Chat list or a Chat window won't unblock them on any other Google properties. Instead, you have to unblock them from within a given Google property. (If your contact is only blocked in Chat you can unblock them at any time by following the steps below.)
Unblocking a contact who is only blocked in Chat:
- Type the name of the contact you wish to unblock into the Search field at the top of the chat list.
- In the box that appears, click Unblock (or uncheck the Block option in Gmail).
Alternatively, you can unblock a contact through the Options/Actions menu at the bottom of any open chat window.
Reference Blocking Contacts