解决方案：
public class ObjectRule : ObjectOverrule
{
public override void Erase(DBObject dbObject, bool erasing)
{
base.Erase(dbObject, erasing);

		//定义数据库
		Database db = HostApplicationServices.WorkingDatabase;
		//获取当前文件
		Document doc = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument;
		//获取当前命令行对象
		Editor ed = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.Editor;
		ed.WriteMessage("\n不能删除！");
	}
}
public partial class Commands : IExtensionApplication
{
	private static ObjectRule objectRule;
	public void Initialize()
	{
		if (objectRule == null)
		{
			objectRule = new PvTools.ObjectRule();
			Overrule.AddOverrule(RXObject.GetClass(typeof(BlockReference)), objectRule, false);
			objectRule.SetXDataFilter("BOM");//过滤
		}
	}

	public void Terminate()
	{
		throw new NotImplementedException();
	}

｝
下面这个是网上找到的参考资料
public class ExplodeOverrule : TransformOverrule
{
private static DocumentCollection acDocs = acApp.DocumentManager;

	public override void Explode(Entity e, DBObjectCollection objs)
	{
		Document doc = acDocs.MdiActiveDocument;
		Editor ed = doc.Editor;

		try
		{
			ResultBuffer rb = e.GetXDataForApplication(Overrule.GetClass);

			if (rb != null)
			{
				Database db = doc.Database;

				using (Transaction tr = db.TransactionManager.StartTransaction())
				{
					BlockTable bt =
							(BlockTable)tr.GetObject(doc.Database.BlockTableId, OpenMode.ForRead);
					BlockTableRecord btr =
							(BlockTableRecord)tr.GetObject(bt[e.BlockName],
							OpenMode.ForWrite);
					Entity ent = e.Clone() as Entity;

					btr.AppendEntity(ent);
					tr.AddNewlyCreatedDBObject(ent, true);

					tr.Commit();

					ed.WriteMessage("\n** Cannot explode custom object ** ");
				}

				return;
			}
			else
				base.Explode(e, objs);
		}
		catch (System.Exception ex)
		{
			ed.WriteMessage("\nerror; SystemException: {0} ", ex.Message);
		}
	}
}