There are several reasons for why a user is unable to remove a star from a constellation in the setup manager.
Usually this is caused by some elements of the dimensions attributes or star quantities that you are trying to remove which are still elements in the system model, but no longer in the setup manager (usually marked as hidden).
This could also be a problem with composite dimensions that contain one of the used attributes from that dimension.
Another cause is the Star is marked as primary in the Constellation, and cannot be removed. Running the below query will make another Star the primary Star, but will break any Stars and Dimensions linked to it.
update Constellation
SET PrimaryStarID = 'EB67DFB3-1B14-4C29-B40A-101620C831C8'
WHERE ID = 4
If the above are not yielding results, in the SQL Profiler, you can try looking at the SQL query that is generated when removing the star and pressing the OK button, which will help give you some ideas for where to diagnose the issue.