How to unban yourself (using FBI)

If you get error code 002-0103, you need to swap your current LocalFriendCodeSeed_B with a public one. You can do so by follow the guide below.
Note that this doesn't cover unbanning from NNID services (like the eShop or Miiverse), which is a different kind of ban.

Step 1: Download the latest public LocalFriendCodeSeed_B file from here (or another source):
http://pastebin.com/jP4WMKYD

Alternatively, use a private LocalFriendCodeSeed_B, which might be less likely to get banned.

Step 2: Put the LocalFriendCodeSeed_B file in your SD card (it doesn't matter where).

Step 3: Launch FBI on your 3DS.

Step 4: Select "SD", hover to your new LocalFriendCodeSeed_B file, select it and then choose "Copy". Press B until you're back to the main FBI menu.

Step 5: Select "CTR NAND" and confirm. Then select "rw", then "sys", then "<current directory>" and finally choose "Paste".

You're now unbanned and can play online again.
The LocalFriendCodeSeed_B you'll be using will likely get banned at some point, meaning you'll have to redo this guide from the beginning.