Why is delete button disable on certification periods for e-learning courses?

Hello,

A number of learners completed courses, which completed certifications, and then were unnecessarily enrolled in the course again, thus they became Subscribed instead of Certified.  The OLM Admins have deleted many of the unnecessary periods, but for the e-learning course periods, the delete button is disabled.  Why is the delete button disabled for e-learning courses, and can we delete them somehow through the application?  We do not want to write a script with an API.

Thank you, Kevin