Sorry I didn't read your post carefully the first time.
If you want to prevent data going from Salesforce to Marketo, change the visibility of the record to be hidden from the Marketo sync user.
Have a read through this document: Restricting Leads from Syncing to SFDC With Sync Filters The idea is having a custom field setup for the contact records and basically allowing Marketo to read the fields that have this custom field marked.
Hope this helps
This is exactly what we have set up on our system to prevent massive amounts of bad data from flooding our Marketo instance.