Standard users cannot access CMS Desk (Site manager) by default. Only users who are either editors (property Is editor) or administrators (property Is global administrator) are allowed to access CMS interface.
Moreover, Logon web parts in Kentico CMS
does not set the Is editor (or Is global administrator) property to true while user‘s registration. So, you can use them for this type of situation.
For more information I would like to point you to the following article: User management