Key in DELETE UNUSED ELEMENTTEMPLATES to delete ALL unused element templates.
or
Key in DELETE UNUSED ELEMENTTEMPLATES <TemplateName> to delete a specific unused element template.
Note: Deletes element templates that are not associated to any graphic element in the open DGN or DGNLib file. If the open DGN or DGNLib file is specified in the
MS_DGNLIBLIST configuration variable then no element templates will be deleted.