Email notifications for Tasks are based on the 'Reminder' on the Task.
If you do not wish to receive an email notification for a Task, ensure that no reminder is added.
To remove a reminder on an existing Task:
Select the Edit icon on the Task
Click the trash icon next to the reminder
Select Save
Additional Information:
Task reminders cannot be removed in bulk. If multiple tasks need to be removed, it will need to be completed individually
Reminders can be automatically added for all tasks that are created from a Workflow. These reminders can be removed from the Workflows settings. This will not remove all reminders for existing tasks, and will remove reminders from any future tasks that are added with this Workflow.