How to block a specific App from accessing my website

Someone has an app (Android) that accesses my website and displays some pages.

I would've been ok with it except that the app has some errors and people using it are unable to use certain functionalities of the site.

How can I block this specific app?

P.S. I have root access to my web server and it's dedicated. I run CentOs 7 and using Joomla for my website.