In a previous post, I wrote that I used the "Norton Removal Tool" and "unyop.exe" programs finish removing Yahoo Online Protection after the initial uninstall failed. Well, I found another possible solution I should have tried first.
The AT&T Yahoo! help pages include an article "How do I uninstall AT&T Yahoo! Online Protection? - AT&T Yahoo! Online Protection version 2.5". I had read that article but I missed this at the bottom: "If you experience any difficulties with the uninstall process, click here to perform an additional uninstall of the software." That "click here" link downloads a file called "yaxclean.exe". I haven't tried it, but at least it's documented, unlike the "unyop.exe" program I ended up using.