Locate the User and click on their username to access their profile (see view Users if you're unsure how to do this).
Click on the Archive/Restore link from the Common Tasks area (you can also click on the Details tab followed by the Archive button if you prefer).
You will be taken to a two-step wizard.
Step 1
Read the warning message.
Click the Archive button.
Step 2
The system will confirm that the User has been Archived.
Click Finish.