Why deactivate instead of delete?
Delete a User if he will never be associated with the website again. Note: this will remove all records of that user's activity in page versions and ownership.
Deactivate a User if she will temporarily lose her association with the website. For instance, you might want to do deactivate a User if she’s on an extended vacation OR if you want to preserve a historic record of what they did on the site.
Steps for deactivating a User
Go to Dashboard > Users > Users.
Click on the Username of the User you want to deactivate.
If you need help finding a user, review the Finding Users lesson.
Press the Deactivate User button.