BBQ Pro makes it easy to block requests reporting an empty or blank user agent. This 5-second tutorial shows how to enable easily with a click.
Ready? Don’t blink or you’ll miss it. To block any/all requests that have a blank/empty user agent, visit the BBQ Pro settings and enable the option, “Empty Agents”. Save changes and done.
Important: while an empty user agent in general is associated with foul play, some sites such as Facebook actually use it for various API-related tasks. So before enabling this option, make sure your site does not depend on any service that makes use of empty user agents.