Here are other ways I would try to ban CFW users if I were Nintendo, aside from the "share information" setting and old banning methods they've used in the past:
- Hacked titles on the Friends List
- ctr-httpwn
- Out-of-region games played online
In the A9LH days, I would have also pushed a system update that wrote an arm9loaderhax.bin file to the SD card on everybody's 3DS. On an OFW 3DS, it would do nothing. On an A9LH 3DS, it would run instead of Luma3DS (since Luma3DS would be overwritten), and its only purpose would be to overwrite A9LH and restore the user to OFW.