In Netsuite there is a field called Inactive that lives on a Contact record. It is either set to FALSE or TRUE. If it is set to TRUE, Formstack Sync will ignore this contact and will not pull it down in to the sync.
We recommend using this feature when you have duplicate contacts and you are unable to delete the duplicate contact for one reason or another. Using this field and setting it to YES for the duplicate contact will ensure it doesn't get pulled down in to your Formstack Sync Data sync and won't cause any issues as we outline in this help article around why duplicates are problematic for Formstack Sync Data.