Within Teamwork.com, you can downgrade a standard user to a contact at any time.
Note: Contacts contain reference information and cannot log in to your site.
Click the More option in the main navigation menu and select People from the pop-out menu.
In the People subsection, click the name of the relevant user to view 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