The handlebar is rotating in the stem, yes?

I'd take the faceplate off the stem.

Then, wipe down the clamping surfaces on both the handlebar and stem.

You might try dabbing on a tiny bit of carbon paste (it's basically a gel/grease mixed with grit--on carbon bike parts, the grit bites into the carbon when you tighten the part to give a little physical resistance to twisting). It's not that expensive but if you go out and buy a tube, it will be way more than you will ever use--when I need some, I usually just pop down to my local bike shop and ask them nicely, and they'll squeeze a tiny bit into a plastic bag for me.

Then, reassemble. Make sure you evenly screw on the face plate of the stem (it's a bit tedious, but you alternate screwing down the bolts so that no one bolt is tighter than the other(s)).