Tasks and Projects in Task Manager can be deleted as long as the Task Manager User has the appropriate permissions.
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Permissions
To have the ability to delete tasks and projects, the Task Manager user must be assigned a Manager role. This can be done by locating the user under Partner Center > Tasks > Users or Partner Center > Administration > My Team, then hitting 
to the right and clicking on "Edit User", then "Manager."
OR
Deleting Projects Inside Task Manager
Once made a Manager, the Task Manager User can navigate to the project view in Task Manager, then click on the dropdown to show all of the tasks within a project. Click the (beside "Add Task"), then select Delete Project.
Deleting Tasks Inside Task Manager
Deleting individual tasks can be done through Accounts > click on the account name > View all tasks. In this view, the Task Manager user can click the checkboxes of the tasks they would like to delete, then a "Delete" button will appear across from the search bar.
Video Walkthrough