First you need to edit the network interfaces file using the following comamnd
sudo vi /etc/network/interfaces
sudo gedit /etc/network/interfaces
Now you need to add the simple iptables rule to the interfaces file when the internet connection starts up
Simply add this under auto wlan0 or auto eth0 in the interfaces file
pre-up iptables -A OUTPUT -p tcp -m owner --uid-owner username -j DROP
save and exit the file.
Now you need to type in the terminal with the following command
sudo iptables -A OUTPUT -p tcp -m owner --uid-owner username -j DROP
and switch users to the username you blocked and try to access the internet.