How do I delete a yahoo question?

You can only remove a question before it receives its first answer.
Go to the Yahoo Answers page where you posted your question.
Click the Edit icon under the question.
Click Delete Question.
Click Delete to confirm.
If answered you can make it anonymous by edit Make Anonymous. This will make it so the question is unconnected to you.

You can't as long as someone has answered it.

It depends. Is it your question or a question raised by somebody else. If it is your own question, cancel it. That is all. You cannot do the same for other's questions.