Currently there is no such functionality available.
The only way to cancel the sent response to the client is by redirecting the page. Also, this should not be in a partial postback, i.e., in an update panel.
As for Q3 2014, there will be a cancellation functionality that will allow the page to not be redirected in teh ExportContent event. You can examine again the RadEditor documentation when Q3 2014 is live.
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