If they are still on your website, block them in the conversation, click on the menu icon (1) and after that select block visitor (2). You can unblock a visitor the same way. The chat will disappear for the blocked visitor.
If you know the IP address of this visitor, it is possible to block the IP in the Settings → General (1) → Blocked IP addresses (2) → Add IP address (3)
Still confused? Contact us via the chat box in your dashboard and we’ll be happy to get in touch with you. Please note that customer service is included only in paid packages.