More effective than Stealth Settings on Yahoo! Messenger if you want to be left alone (not to receive messages from an ID) is option which can reduce a ID to silence. ignore.
Messenger (from the Toolbar) -> Preferences -> Ignore List (Category) - Check the option “Ignore only the People Below”-> Click on the button ADD -> Write IDWhat's to be ignored and select the version he used chat. -> Click on ignore -> Apply -> OK .