You didn't mention how long it has been since the pre-approval request was approved. If it's been more than 24 hours, the pre-approval has expired (and, thus, you can't rescind it because it's no longer in effect). Yeah, the message thread with the requestor will still show a (green, I think) "pre-approved" label, but that's simply stupid design on AirBnB's part; don't sweat that.
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