User accounts can be either marked as deleted (keeping all the associated data) or they can be deleted permanently. In both cases, you can delete a user from the User Management page. First find the user account you want to delete and then press the delete button.
Once you press that button the system will show you a confirmation dialog.
Press "Delete" to mark the user as deleted. In this case, the user account will be marked as deleted but all the associated data will remain available in the system. You can easily revive the user account if you need to following this instructions: Finding and undeleting user accounts
To permanently delete a user account, select the "Delete permanently" and follow the instructions.
Note that before deleting a user permanently you will have to eliminate any records associated with that user (Goals, Feedback, Journal entries, etc).