Delete ISBNs for formats other than what is being described by the bibliographic record. For example, delete the ISBN for the ebook from a record for the regular print version. Also, delete ISBNs for other volumes in records for an individual volume of a set.
The ISBN for the paperback may be retained in the record for the hardback if the paperback meets the OCLC Bibliographic Formats and Standards match criteria for the paperback and hardback to be attached to the same record. See also the SHARE cataloging standard: Paperbacks and hardbacks on the same record.