Ah, I see. In that case it would be a matter of evaluating how well the shim fits in the stem saddle and how well it fits the bars. If it's still turning then you either have a slightly loose fit or you're just not tightening it enough because you're afraid of bruising or busting the carbon.

The shim should need to be pressed onto the bars. It should seat correctly but only with a firm push from your thumbs. If it doesn't then it's too loose. A shim of pop can metal or something thinner may be required to achieve that sort of fitting. Similarly with the shim on the bars it should take an equally firm two thumb push to seat the bars and shim into the stem. Again, if it is loose then you're only going to get solid pressure at the most fore and aft points.