Once the coins left ypool there is not much that can be done. But in this case you were lucky and he still had the coins on his pool balance. Therefore I have transferred the coins back to your account.
Please change your password immediately if you haven't done so already. Make sure you set it to something that you don't use on any other website and that can't easily be guessed.

I'd like to remind everyone that we have quite an extensive security system setup on ypool. If you set a secret question and/or 2step authentication it becomes impossible for someone to steal coins even if he has full access to your account.

Best regards,
jh00 from ypool.net