A new feature has added to admin panel to allow agents to block access of certain clients. It works based on client's IP address and a unique cookie that sets on each client.
To block a client click on 'Block Access' in chat panel conversion. Another section has also added to 'Chat Panel' section of configuration menu to manage blocked clients. This will be accessible by root users only.
http://www.liveadmin.net/forum/viewtopic.php?f=6&t=13