How can you stop 50 user's from using internet in an organisation where 200 employees are present and has a domain(windows server) environment setup?

Jiyad, create a group policy in your windows domain server in such a way that  change your internet explorer settings by add the proxy server address as your local address(127.0.0.1).

Please feel free to  get back to me if you have any further questions

We can restrict Internet based domain users also through TMG

suggestion 1:create1 organization unit for example blocked users

                   -create group policy and reconfigure IE settings by hide the proxy settings and reconfigure the proxy IP with fake address.

suggestion 2:if you have DHCP server you can reserve the IPs for the users that you want to stop internet for them, then it depends on what you have (router or Firewall) to block their IPs.

This work around solution but if you can get web filter solution (TMG, Blue Coat,Sonicwall,etc...) for sure it will cover your requirements.

Block internet access in the respective users accounts, preferably account settings > Control Panel etc.

As employee Internet abuse has become an identifiable concern, corporations have developed

various strategies to help combat the problem. First, employers have utilized Internet Use

Policies to set forth written guidelines on acceptable/ unacceptable Internet conduct. The

Internet Use Policy is a visible tool that not only provides guidance on appropriate online

behavior but also outlines how violations will be handled. According to the Society for Human

Resource Managers, attorneys advise companies to write policies on e-mail and Internet use and

electronic monitoring procedures. They also advise employers to regularly alert

employees that their online activities may be monitored and that inappropriate use may result in

disciplinary action. A growing trend suggests that a number of corporations rely upon Internet

use policies to cut recreational use of the Internet during work hours and to mitigate legal

liability regarding such misuse

you have many option to prevent users using the internet like :

from firewall (device , software) you can prevent them from accessing the internet.

also that you can prevent user from gain access internet through Group policy .