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)
Any questions? Contact us at firstname.lastname@example.org