For Basic Elements this is done by selecting Non-editable by CMS checkbox in the element's Properties Panel. 

This will prevent your content manager from editing that element, keeping it's content and design intact.

For all other elements, a CMS Access dropdown field is shown at the bottom of the Properties Panel. 

Click on the dropdown to reveal the different options available to you. 

Show All Properties: This will give your CMS user full access to edit the element.

Hide All Properties: This will block your CMS user from editing the element.

Manage/Link Only: This will block your CMS user from editing the element, but still allow them to access the Manage area or Link option for specific elements. For example, Galleries have a Manage section. This would allow the CMS user to still be able to edit the images, titles, descriptions and links to the gallery but not to edit the design itself.