For Msn messenger / Windows Live Messenger, we can easily click on the "Block" button to block and report the people is a spammer. But on Yahoo messenger, there is no "Block" button. When the virus or spammer send a message to you, you must not close the conversation window yet, you can click "Delete" button to delete him from your messenger. Then tick "delete from Adress Book too" :

Then only the "Ignore" button will appear on the conversation window, so you just click the "Ignore" button to block him and report him to let yahoo company know he is a spammer / virus :