How can I prevent a none root user to executes e.g. the
passwd command in terminal?
To avoid a user from running the passwd command, just make it non-executable for everyone but root.
An odd request though. You usually want users to be able to change their password, and to do it regularly. Also, the passwd command isn't the only way a user can change their own password...
With sudo you most certainly know that the default setup will require the user running sudo to enter a password (by default the password of the user running sudo and not the root password). In Ubuntu, if you wish to remove a user from the admin group, run this command in a terminal:
Check it out:
You should see something like that:
without the admin privilege.