It's a shame to destroy the stem, but I understand your reasoning.
My first step would be to find a less snooty bike shop willing to do the job. Or seek out a local bike co-op.
Otherwise, you can do the job your way as follows.
Wrap cloth tape around the bar on both sides of the stem as indicators.
Now use a hack saw to cut most of the way through the stem without touching the tapes. Remove the clamp bolt and pry the stem open until it clears the bar or snaps.
BEFORE destroying the old stem, MAKE SURE your stem's clamp diameter matches the bar. There are multiple standards, and you don't want to end up stranded.