Removing a task or task list is the same as deleting a file or page from a workspace, and so it can only be performed by users with "Administrator" or "Editor" permission in the workspace's People list.  Please note, there is absolutely no method to recover a deleted page or file.  First, access the workspace the task or milestone is in and click the "Tasks" tab at the top.

Deleting individual tasks:

To delete an individual task, double click on any task list to view the tasks associated with that milestone.  Then, click on the task you want to delete.



Finally, click the "Task Options" menu in the upper right corner, and then "Delete" to delete the task. This removes the entire task from the task list, and all links to the task will be broken.  For completed tasks it is best to simply mark the task as complete, rather than getting rid of it forever.