I have had this exact same problem with this exact same stem. Loosen the stem bolt but not all the way out- there should be one or two threads still in the bottom 'cone' part. The tap the top of the bolt head- JUST THE BOLT HEAD- using a socket extension or similar metal part to ensure that ONLY THE BOLT HEAD is being struck. Use a metal hammer to tap it- not a dead blow. The cone should drop right out and presto- stem-O-remove-O.