If you've banned a visitor from chatting, you can undo the ban at any time.
Go to Visitor Tracking and open the Banned Visitor List at the upper right corner of the screen.
In the list, click on the customer you would like to unban.
Click Unban at the bottom of the customer information panel. The customer will immediately be able to see and interact with your live chat widget, and you will be able to view their activity in Service > Visitors.