You can't delete a user in AGOA. Instead, deactivate the account. The user loses access right away, and all the data they entered in the past stays in the system.
How to deactivate a user
Go to the Backoffice and open the Users section.
Find and open the user.
Turn off the Active user toggle at the top of the page.
For the full steps, see How to activate or deactivate a user.
What happens when you deactivate a user
The user is signed out immediately, even in the middle of a session, and cannot log back in.
Their past data (timings, comments, reports) is kept, so your history stays complete.
You can reactivate the account at any time.
If you don't see the option
You probably don't have the required permissions. Check your role, or contact your administrator.