User accounts can either be marked as deactivated (in which case all of their reviews/data are retained) or they can be deleted permanently.
In both cases, you can delete a user from the Users. First, find the user account you want to deactivate/delete and press the Delete button.
After clicking Delete, a popup will appear asking if you want to deactivate or delete.
*Note: In the vast majority of cases, you will not want to permanently delete your user. Simply deactivating them will prevent them from logging in in the future but allows you to retain their data (i.e. previous reviews, goals, etc). Deactivating a user does free up a license to be used by someone else.
If you choose to permanently delete someone, the action is final and cannot be undone. The system will not allow you to permanently delete a user until all of their associated data is first removed from the system (i.e. you must delete their previous reviews, goals, journal notes, instant feedback -- everything) to permanently delete.