I immediately put all withdrawals on hold, even small ones, banned him and then disabled the site. So after that he couldn't do any more damage, but he still managed to withdraw aforementioned 182 BTC to his private BTC address.
I hope this explains it all.
OK, so since I notified you less than 10 minutes from the sale, does this mean that you have been able to stop him from withdrawing my 15 BTC? As far as I know it takes at least an hour to get a few confirmations and be able to get the coins...
No. The minute your Bitcoin client announces a trade, you can't revert it. Just a minute after this it has already propagated throughout the network and other Bitcoin nodes acknowledge it. The "one hour" thing is a Bitcoin security measure for other Bitcoin clients to ensure that your "announcement" wasn't forged, i.e. that the coins you claim that you're spending really belong to you.
15 BTC is not a lot, but it's not a small amount either, I understand you. I've spent around 15 BTC (in my currency though) to buy my first SSD drive few weeks ago. Believe me, if there was anything I could do to stop or revert this, I would have done it.