Navigate to the "Manage Users" page and click on the user you'd like to remove. This will open a window with a detailed view of the user's setup. At the bottom right of the window, there is a button labeled "Deactivate User". This will remove the user's access to Fidesic, and you will no longer be billed for them.
If you ever need to reactivate a user that was deactivated, from the "Manage Users" page select "Options > Inactive Users". This will load a list of any inactive users, with the option to reactivate the user on the right side of the screen. This will allow the user to log back in with the same email address and password they used at the time they were deactivated.
If the user is not sure of their password after having their account reactivated, they will need to reset their password.