Limit Fivestar for node author only
Here are the steps you need to allow only the node author to rate the node.


For the Task content type, change the role permissions so that only the authenticated users (or whichever role you want to allow to complete and rate Tasks) have the "Edit own content" permission.
Under "Manage Fields" for the Task content-type, configure your rating/fivestar field to use the widget "Stars (rated while editing)". Now, only the node's autor can edit and rate a Task node. The vote will still be visible to other users.


Hope this helps!