While recovering from a corrupted SD card on his 3DS, YouTuber Anthony Schwader discovered a means to delete Nintendo-issued mii visitors and respawn them endlessly. It's actually pretty simple.
It works like this—use your special mii character, such as the Abraham Lincoln mii sent to promote Code Named S.T.E.A.M. in all the StreetPass mini-games you own. Turn off StreetPass and SpotPass. Delete the special mii. Close the StreetPass Mii Plaza. Open it again. Reactivate StreetPass and SpotPass and wait for the special mii character to pop back up.
Anthony passed along a video of the exploit in action, and says he is "Totally stoked that it might help people in more rural areas whenever they release a new Guest Mii!"
Here are the steps once more, as provided by the man himself.
1. Use the special guest in all of the games
2. Select SETTINGS and deactivate STREETPASS (This also deactivates SPOTPASS)
3. DELETE the special guest from your plaza
4. Hit HOME and exit StreetPass Mii Plaza
5. Re-enter StreetPass Mii Plaza
6. Select SETTINGS and reactivate STREETPASS
7. Reactivate SPOTPASS next
8. Wait for the beautiful BLUE NOTIFICATION
9. Hit the HOME button and the NOTIFICATIONS bubble should pop out.
10. Enter NOTIFICATIONS menu and select LAUNCH SOFTWARE at the bottom
11. SP Mii Plaza will close and re-open by itself and the special guest should come strolling through to greet you like it was the first time ever!
12. MISSION ACCOMPLISHED: SPOTPASS BROKEN - REPEAT UNTIL YOU ARE SATISFIED