Right now I got original IBurn from SDK and add one line in function BurnDirectory(line 82). Got fresh erased CDRW media and burn few files. In result I have finalized media. May be you have tried to set ForceMediaToBeClosed in wrong place? You have to do it before Write.
// Write the image stream to disc using the specified recorder.
dataWriterImage.Write(stream);// Burn the stream to disc