I believe you'll have to go into said assembly and remove the part from the assembly. Then you should be able to delete the part from the workspace or common space.
this does not always work, especially with formats and drawings, PTC recomends utilizing the purge manager to remove the unwanted, non-latest iterations that is responsible for the referential integrity, unfortunately I haven't had success with that either.
I have faced this situation ,where the deletion of object is blocked with "integrity violation" exception.
This is an excpetion thrown only when an delete action is performed on WTObject , which is linked with any other object.
The only soution to overcome this is to break the LINK(WTPartUsage/WTReferenceLink/WTPartDesc Link/Configurable Link) between the object to delete and the other side object.
Try using a Listener /Event Rule class with a PRE-DELETE Event,which will break the link and the proceed with Windchill's delete.
