Someone has invaded my address book and is sending emails that look like they came from me - how do I stop it?

This was probably caused by someone obtaining your ID and password and copying all your contacts. From that point on they do not need your account. Anyone with a separate email program can use any address as the FROM address. There is no way to stop this method. They can continue even if the account is closed. They already have the list. To make sure they are not actually in the account change the password. This is not normally caused by a virus. In the rare case where it is then the virus is in your computer and not at the Yahoo mail website. Changing your account may not solve the problem since any virus is going to get access to your new account too. ℬ ℋ

If it was me I would immediately register a new Yahoo address.. and then send an email to everyone currently in my address book informing them that you had been hacked and that spam will arrive showing your address.. ask them to block the old address and to use your new address... Good Luck...

If it was me I would immediately register a new Yahoo address.. and then send an email to everyone currently in my address book informing them that you had been hacked and that spam will arrive showing your address.. ask them to block the old address and to use your new address... Good Luck...