Hi Roberto,
The 'assign moderator' button is only visible if the usergroup has moderation permission in the post's category as configured in the category(http://take.ms/zFXl6). All super users are also moderators by default. So if you do not want the user group to choose a moderator, just remove the permission for those usergroups.
Also, the 'assign moderator' button is just to assign users with category moderation permissions to a post. It does not give regular users moderator permission if the user group does not have moderator permission in the category.

As for moderator actions, you can set them individually in the ACL section(http://take.ms/uWHz0). This applies too all users in that usergroup.