A bug preventing Google AdWords advertisers from canceling their accounts has finally been fixed.
The issue was first reported in a Google AdWords Help forum back on December 13th or before and confirmed by a Googler then. But even though it was considered a "top priority" it took two full weeks to resolve.
The bug was posted on the known issues page on AdWords and it said, "Some advertisers could not cancel their Adwords account. This issue has now been resolved by our engineers." I am not sure if this was resolved on December 17th, four days after it was confirmed, because the AdWords representative in the forum reported it as fixed this morning.
Forum discussion at Google AdWords Help.