Most likely you are describing a spacer but is it not clear.
The spacers should not spin. If they do its a sign the bearings have little or no pre load and the head set will be loose.
Couple possible causes
1) the pinch bolts where not loose when the top cap was tightened
2) you are missing a spacer
if you want to lower the bar you move the space from below to above
someone may have just removed a spacer.
Internal view of head assembly
Note: There must be a gap between the top of the steerer tube (fork) and the top cap. If not, the cap bolt cannot pre load the bearings and a spacer needs to be added. It can be added above the stem if desired. Alternately the steerer can be cut shorter.
How to fix:
(Should take no more than a few minutes)
- Back off the cap bolt a couple of turns. Loosen the pinch bolts on the stem till it will easily rotate.
- Tighten the top cap bolt (not too much - this sets the bearing pre load.).
- Tighten the pinch bolts.
If this does not work
- Leave the pinch bolts tight and remove the top cap
- Check for a gap between the top of the stem/top spacer and the top of the steerer.
- No gap - go to LBS and get a spacer or two (a few dollars at most).
If there is a gap, and the spacers below the stem are still turning, something is very wrong. Check the threads on the bolt and star nut are not the problem.
Refer Here for instructions