How To Remove User from Learning Plan (Retain Mapping was Enabled but LP No Longer Mapped to Group)

Summary:

Has anyone ever had to remove a user from a learning plan that was assigned via a group with retain mapping enabled AFTER the learning plan was unmapped from the group?

To be clear, I know how to remove users from a learning plan when they're assigned via a group with or without retain mapping selected. I'm stuck because the learning plan is NO LONGER mapped to this group so I cannot uncheck retain mapping to remove the user and then enable it again.

The group is mapped to other learning plans, but not the one I need to remove the user from.