Thank you, Adam. That article is a great place to start, but what I need to do is disable SalesForce Sunc for a specific list that I am about to upload to Marketo but have the stuff already in SalesForce continue to sync.
If you create new leads in Marketo they will not auto sync to SFDC. They have to go through a MArketo campaign to push them. Just make sure they dont' got through one of your campaigns