To remove a stem that is tight, I use what I have.....Plastic shims, large flat head screw driver, putty knife or a combination of them being careful to not mar or damage the stem.....some bars seem easier to un-install than others, especially at the bends.
When the bars are going to get stuck, I don't continue to push and spin them, what I do is to back off and rotate the bar until I find the path of least resistance (there is one!) and then the bars and the stem seem happy and will uncurl/slip away with no damage.
I can't give you the exact angle/rotation, but with care and a bit of patience, you will find the sweet spot...you don't need grease, tape, or paper, but use them if you feel the need. Practice/practice at the factory where bars were installed helped workers find that spot.
Some may find it easier is to remove the stem and bar from the frame.....it's easier to spin the stem and bars...either way take your time.
I hope this helps, Best, Ben
wide flat easy no mar etc