Change your password to something hard to guess. Change your secret questions (The questions Yahoo would use to help you if you forget the password). If it is a case where everyone on your contact list keep getting an email from your account, it is fairly common. Usually it is a link which takes them to a pharmaceutical site promoting viagra, etc. Changing what I mentioned will stop it.
First do a little home work ,check your sent file to see where the mails are going.Then try abuse@yahoo.com give detailed report.When you leave yahoo dont just close the the operation sign out. That shuts the door. Check for cookies you may have a key stroke bot giving your information away.Good luck I know very well how much this sucks.