In order to delete your Game Sync ID, we would need to confirm your registration. Please provide the following information by submitting a question at support.pokemon.com:
- Trainer Club account username
- PGL nickname
- Registered DOB, starting with the year
- The country you live in
- Current Game Card in use
- Currently registered Game Sync ID
> When deletion of the registered Game Sync ID is completed, we will reply to your question. Then please register your new Game Sync ID.
Source Good luck getting it deleted!