I was at a convention a couple months ago and temporarily friended someone for the sake of Mario Kart races. I deleted him shortly after returning home, since he's not actually my friend, and I didn't want him in my friend list. In the last month, I've received three Swapnote SpotPass notifications saying that the Swapnote either contained invalid data (unlikely to happen three times in a row), or that it came from someone who is no longer a friend (much more likely, since he did send out a Swapnote to all his friends before I deleted him). I don't want to keep getting SpotPass notifications, though, only to find out it's for a Swapnote I can't read. Is there a way to stop these notifications from occurring, while still getting notifications for legitimate Swapnotes?
In short: friended a guy, deleted him, don't want to get blue light notices for his Swapnotes anymore.
"Cats...are kind of like girls. If they come and talk to you it's great. But if you try to talk to them it doesn't always go so well." --S. Miyamoto
If you can remember his FC, re-add him, send him a note asking him to delete you from his friend list. Then delete him again. There's not much else you can do except for blacklisting him - but for that, you'll need to have his FC again.
"Evil, beware. We have waffles..." - Raven, Teen Titans