In this production release (November 17), account admins have access to a new permission that controls the ability to manually mark learner course enrollments as complete. (Click an image to enlarge screenshot)
Override Enrollment Data Permission
This new permission allows account admins to control who will have the ability to manually mark a learner’s course enrollment as complete. The Override Enrollment Data permission can be found under the Manage Courses permission group in the Content section of the Roles and Permissions page. The permission will be enabled for account admins by default.