I am asking this option because of some useless answers I got for my question.
I can down-vote those but there is no use.
If I can delete that useless answer, my question will be noticed as unanswered question and I may get better answers.
I flagged the answer to be deleted, but nothing happened.
SO has an option to close the question; why don't you add an option to close answers, or make it not considered an answer.