You cannot permanently delete a user from the system. This is so that the system can retain historical data, related to item creation and approval processes. This process will show you how to disable a user's access.
- Navigate to the User Menu in the top-right corner
- Select Site Settings
- Scroll to Users in the Users and Security section
- Find the user you want to disable access for
- Click edit tool next to user
- Uncheck Enabled option and select Hide from Staff List and Hide From Approver's List while in User Settings
- Select Save at the top of the page