There is a purge function provided on the purchase request form that will allow you to remove the history. As the send status is a record of what you have communicated to a supplier you probably don’t want to just delete it. But you will need to clean up the amount of records you are storing live in the database to avoid growing the database unnecessarily with old data. You will need to provide a path and file name in the cXML parameters form. When you use the purge button it will take the records you have selected and export them to the archive file and then remove them.
Here is a quick overview.
Refer to this post for an overview of the cXML Purchase Order Send.