In order for you to Ban Users from My Web Site for CometChat?
Login to CometChat admin panel.
From Left Menu Click on "Settings->General"
Search for "Banned Words and Users"
Search for "Banned User IDs:"
Enter specific users id which you want to ban. Change these Settings as per your requirement.
Clicked on "Update" button.
"Clear Cache" from the Admin Panel.
Clear Cache from the browser.