What is the way to have MANAGER only allowed to delete or mark a task as completed for closure?

What is the way to have MANAGER only allowed to delete or mark a task as completed for closure?

I just discovered that any EMPLOYEE level user can do the following:

 
1.       Close a task assigned
2.     Delete a task assigned

This has serious implications on usability and security, as one task may be assigned to multiple users. If any EMPLOYEE can delete a task, all the related time sheet entries of all others assigned to the task also get deleted.

What is the way to have MANAGER only allowed to delete or mark a task as completed for closure?

Thanks and regards,