Within Teamwork Projects, you can choose to disable emails individually for each person so that they cannot be selected in the notification options.
To do this, go to the People section of a project.
Next to each user on a project you will see an envelope icon. This will be green by default which indicates that they can be selected in the notification options.
Click this and it'll turn red, to show that their emails are disabled.
You can click it again to re-enable the emails.
With this disabled you will now see the user listed with a red X beside their name.
Note: The People tab in a project is accessible only to administrators and projects administrators
For more information, see: Project notification settings