As a Great Teaching Coordinator, you are able to deactivate users in your organisation, and then you can re-use that licence. Simply head to the “Teachers” page of your account, hover over the three dots in the right-hand column, and click on the "Deactivate account" option.
Beware: if you deactivate a user, they will lose their access to all courses and some of the other functionality, so make sure you’re certain before you go ahead!