It takes some customizing to do this:
- Create a "Do not accept from Mkto" SFDC field on leads and contacts. Make it a boolean field on lead object, and a formula (boolean) field on contacts, whith formula value = TRUE
- Create a workflow on the lead object in SFDC that checks that box on lead creation, excepted when the creator is the Marketo sync user
- Check that box for all existing leads in SFDC (using a data loader). Wait until this change is reflected in Marketo.
- Create a validation rule in SFDC that prevents the lead creation if the box is unchecked and the creating user is the Marketo Sync User
From this, you will be able to control which new lead do not flow to SFDC by checking the box in Marketo lead management smart campaign when you want the sync to occur.
Remember that the existence fo this checkbox field and the validation rule will prevent implicit syncs to happen...