How to unban your 3DS

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:
http://pastebin.com/jP4WMKYD

Step 2: Put the LocalFriendCodeSeed_B file in the "files9" directory of your SD card. If it doesn't exist, create it manually.

Step 3: Launch Decrypt9 on your 3DS. This is usually done by holding Start while turning the console on.

Step 4: Select "SysNand Options", then "System File Inject" and then "Inject LocalFriendCodeSeed_B".

Step 5: Follow the instructions on the screen.

You're now unbanned and can play online again.