There is no way to delete metadata for an item within ArcCatalog. However, metadata for file-based items can be deleted with Windows explorer. Metadata describing items in a geodatabase is stored as a blob of XML within the geodatabase in the administration table GDB_UserMetadata; contact your database administrator to delete metadata for an item.
Navigate to the folder in which your data is stored. There, you will find an XML file that is associated with the item.
For example, the metadata for a shapefile named landuse.shp is stored in a file named landuse.shp.xml. The same rule is used to name the metadata for other file-based datasets. Metadata for coverages, grids, and tins is stored in the dataset’s folder in a file named metadata.xml. Metadata for an INFO table is stored within the INFO directory. Metadata for a folder can be stored in the folder either as a metadata.xml or metadata.htm file.
Delete the XML file to delete the item's metadata.