A visitor can be blocked for 7 days. To do that, you need to follow these steps:
1. Click on the chat's menu (the three dots on the upper right corner of the chat window)
2. Click on the 'block visitor' option
3. Type in a reason for the blocking in the field on the bottom
4. Click on the "Block" button
5. Click on "OK" on the "Visitor Blocked successfully" message to continue with the other chats.
Once that is done, the chat will be automatically closed. The chat button on the website will disappear for the visitor.