Recently spam hackers have been hijacking Email accounts to use for their own dirty deeds and scams. The best thing to do is to deactivate your Yahoo account and make a new one.
to go to the Yahoo compromisation web site and follow the instructions to disable your account.
The reason deleting the account is because these spam hackers are sending unwanted advertising to all your contacts claiming to be you. As far as you know they might even be sending emails to all your contacts asking for money.
There was a recent scam where spam hackers hijacked this lady's account and sent a message to all her contacts that she was stuck in Namibia and needed money to be sent to a specified account to get home.
To stop it, delete your email account, or if you are able to regain access, change your password to something a little more complex and define for yourself a few security questions only you would know. Another thing is just to contact all the people you had in your contacts and tell them that you account has been compromised and that they must ignore any emails received from it.