A user cannot be permanently deleted from the system. This is because a lot of data within the system associated with this user e.g their documents, forum messages, emails, notes, time sheets, status updates etc. are part of a company's knowledge base and must be preserved. You can archive users. This frees up their license (if they are licensed).
From the toolbar click →
Under general section click
Select user from list and click from action dropdown
If user has projects assigned then reassign the projects to other users and then archive the user.
If user has tasks assigned then reassign the tasks to other users.
