Can I unassign a course from a learner that has left the company and assign it to a new member of staff?

Yes, only if the learner didn’t start or open the course.

First of all un-assign the learner from any courses they haven’t opened by downloading their learner report and removing them from the courses they have been assigned, this will then credit the course(s) they have not opened or started back to your account, which you can then assign to another learner.