Cervelo actually uses an aluminum inner sleeve glued inside the carbon steerer tube, with a star nut. Seemed a little strange to me, and made cutting the steerer tube a little more work, but I guess it works fine. Their manual also states no spacer needed above the stem.
those sleeves are often found on pro bikes--you epoxy it to a (cut) steerer, and it stiffens up the front end.
i've done that with a few bikes and it is a noticeable change.
star nuts aren't generally used on a carbon steerer (unless you have a metal insert); expander plugs can be used on steerers of any material.
did you cut the stem with the sleeve in place? ugh. it's not really different than cutting a steerer as normal, but it's unfortunate if that was glued into place before you chose the steerer length.