To block contacts, right click the contact you want to block then “Privacy Settings” and “Block Permanently” from the menu.
Blocked contacts won’t be able to reach you or see your online status at all, and they will be collected in a specific contacts category as you can see below:
To unblock the contact you need to right click on it and select “Add to Contact List”.
You will send a new friend request to the blocked contact. As soon as your request is approved the contact will be deleted from your blocked list and added to your friend list.