how can I prevent that a member of a group can change his group subscription?
https://support.google.com/groups/answer/9792489?hl=en
I want him to receive all group emails. he must not be able to select "Digest", or "Abridged" or "No email" settings
I tried by API group settings https://developers.google.com/admin-sdk/groups-settings/manage?hl=en setting showInGroupDirectory as false, but users still see the group in the list
Anything you try will fail because you have Groups Service enabled. You must disable Groups Service for your domain: https://support.google.com/a/answer/182442
Only then, you will have control on users with Groups Service disabled P.S: Any domain with Groups Service disabled will have them treated as mailing lists.