Within Teamwork, you can downgrade a standard user to a contact at any time.
Note: Contacts contain reference information and cannot log in to your site.
Go to the People area of your site and select the relevant user to open their profile.
In the user's profile, click the options button (three dots) in the top right and select Downgrade to a Contact from the dropdown menu.
You will be able to remove the relevant person from all projects. You also have the option to unassign the user from their items or reassign their items to another user.
After a user has been downgraded, you can upgrade them again.
For more information, see: Upgrading a Contact to a User