how can I turn off emails at the task level? I have a number of group tasks that

Task Notifications are global, if disabled at the server level forum.appian.com/.../Configuring_Notifications then no tasks will be sent at all.

If the people in that group shouldn't receive assignment emails at all from any model then you can achieve that by editing their alert settings using the Portal Utilities plug-in forum.appian.com/.../summary and its Edit Alert Settings For User smart service. There's a sample app at the bottom of that link, but that will basically disable email notifications (task assignment) for any task for any user within the given group.