There are always complaints from Google AdWords advertisers about not being able to be refunded for one reason or another. A new Google AdWords Help thread reports the issue in detail, with the use of a video.
Google said there is a bug with refunding advertisers who have used pre-paid account. AdWordsPro Sarah said:
The reason you are unable to cancel your account is because there is a bug affecting some prepay advertisers that prevents us from issuing a full refund when you cancel your account. We are aware of the issue and should have it fixed shortly- at which time you can close your account and get your money back.
About 1 minute and 15 seconds into the video it shows the issue with getting a refund:
Again, this is a Google bug they acknowledge. They said they will fix it soon, but there is currently no ETA for that.
Forum discussion at Google AdWords Help.