Problem:
A “normal” user is able to access the users management and can change their own quota or manager other users
Answer:
Its very likely that you have given this user the “Group Admin” permission. Informations about this “Group Admin” is available here:
https://owncloud.org/ownclouds-new-group-admin-feature-whats-it-all-about/
To remove this “Group Admin” permission you have to make sure no group is chosen in the “Group Admin” dropdown menu as shown in the attached screenshot.
