sing the guide to build plugins for Roundcube, I set up a "no_forward_for_groupes" plugin, in the [roundcubeRoot]/plugin/no_forward_for_groupes folder. I activated it in the [roundcubeRoot]/config/config.inc.php file by wrtiting

$config['plugins'] = array('no_forward_for_groupes');

as any other plugin.

After a bit of reading and research in the [roundcubeRoot]/program/js/app.js file (the core JS of Roundcube), I found the object I need to act with and the events to listen. The final code is just after. As you can see, I disabled a lot of other commands, as well as drag and drop. So basically I have a read-only folder, which you can't get any mails out of. I know it's kind of a specific use-case, but I hope it'll help some folks out there someday!