After much annoyance -- Spotlight, which itself has gotten worse in recent releases, does not find the culprit files despite their containing the search string in the filenames -- I believe I have solved this problem. YMMV, works for me, etc.
First, go to Settings and set the Background Items the way you want. This may require some digging since the new Settings app is a UX faceplant, but hey, you'll get there.
Next manually delete, from your Applications area, the offending apps if you can.
Then do a reboot just to be sure.
Now, when you see alerts like the one shown below, go check in two directories:
~/Library/LaunchAgents
Here you may find gems like com.spotify.webhelper.plist and you can simply remove them.
/Library/LaunchAgents
Here you may find things like com.canon.usa.EWCPairingService.plist and com.canon.usa.EWCService.plist -- these require admin removal, meaning "sudo rm" if you're in the Terminal or "use password" if in Finder.
I personally recommend zapping these in the terminal, because I don't fully trust Apple to not give them some weird zombie powers to exercise from the trash.
That's all for today. Just wanted to put this out there in the Internets. Fucking Apple, man, really.
