Solution 1:

First you need to edit the network interfaces file.

Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below.

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 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.