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.