This how to will show you how to block internet access for a user, users or computer within an Active Directory Group Policy Object. I’ve tested this on Windows 7 and Windows 10 and it works great!
There are plenty of tutorials out there detailing a way to block access is via enforcing a non-existent proxy. This method will work for some things, but the problem is not all software necessarily uses these settings to connect to the internet and doesn’t necessarily stop a determined user or bad guy.