Hi Stefano:
One way to do this would be to do the following:
Ensure that the user with Administration / Access Rights does not have Extra Rights / Technical Features privileges. This will prevent the user from using the Settings > Users & Companies > Groups menu item to add a user directly to a security group.
Customize the "Users" form and make the Administration field accessible only to users in the Administration / Settings group by adding a groups="base.group_system" attribute to the field.