To block someone, the person should not be on your friend list. If they are, than you need to delete them in order for you to ignore them. Simply type the name on word or notepad, and then delete the user. Once he or she has been deleted, go to Messenger------>Preference-------> Ignore List------------> and type the person's name and click on "add" and then "apply."
The reason why you should delete the person first is because Yahoo Messenger will not let you ignore someone that is on your buddy list.
