How to remove imessage from ipod from computer [duplicate]

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• Steps after buying a used iPhone 2 answers

I sold my iPod on Gumtree and the guy that wanted it gave it to his 9 year old sister for Christmas.

On Christmas Eve it got a phone call from him and he said that he was still receiving messages that were being sent to the iPod - meaning he is receiving all my messages and personal info... Worrying.

My iPhone I currently have is off as I lost my charger; so I need to solve this issue ASAP via my computer. How do I do this?

Any help is appreciated. Thanks