If you are using the native Google Talk app, blocking a person from messaging or seeing you when you are signed in should be fairly easy and straightforward.
However, if you are connecting to your Gtalk via 3rd party apps, e.g, Mac’s Messages, it can be a little challenging as the Blocking option is nowhere to be found.
Recommended Reading: How to Know Who Blocked You in Google Talk
Blocking a contact
To block a contact, here’s what you need to do:
1. Login to your Gmail.
2. Look for Chat on the left sidebar. Make sure you are signed in.
3. In Chat’s search box, type in the name of the contact you wish to block.
4. Hover the name of the contact , look for his/her’s profile.
5. Look for the arrow button (More options) on the far right, click it, and select "Block"
That’s it! Your blocked contact will not be able to communicate with you across all Google’s network.
Unblocking a contact
Unblocking a blocked contact is also fairly simple:
1. Type in the name of the blocked contact in Search.
2. Hover the name, click to uncheck and disable blocking.