Having assigned a material to a body, how do you remove it?
Select Tools --> Materials --> Manage Materials.
The dialog defaults to Library Materials. The currently assigned material is identified by the green check in the Used column.
Change the pull down at the top of the dialog to Local Materials to see a list of materials used in the part.
Right click the material to be removed and choose either Remove Usages, which removes the material from the assigned bodies, or Delete, which removes the material from the Local Materials.
Notes and References
OS Version: 764SP1