How can I block internet access?
is there a possibility to block the internet access?
That feature will eventually be available in a subscription version of Veyon next year.
In a Ubuntu 18.04LTS environment (Veyon 4.3) you can achieve that goal by means of scripts acting on the firewall (iptables) of the local machine. One script is used to create the desired rules to block/modulate internet access and the other to flush all the applied rules, thus giving full access to internet. You should also modify the sudoers file to give the client users the privileges to execute the scripts with root privileges without being prompted with a password request.
Finally in the veyon master you must add both scripts in the list of the executable programs, so that you can remotely launch them in the clients.
One limit of this approach is that an expert user could find the scripts and get internet access launching the unblock script in the terminal.