Follow these steps to block a user:
From the left sidebar navigate to the Conversations section then click on All Chats tab to see all the users, as shown below.
Select the user you want to block and click on the chat options button on the top right side.
From the options menu, click on Block to block the selected user.
After you block a user, he/she won't be able to reply or type in the chat box. If the user visits your website again or refreshes the page, he/she won't be shown the chat widget until unblocked by the admin.
To unblock a user follow the same steps mentioned above.