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
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.