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