Hi Jason,
The best way to do this is via Sharing Rules in SFDC. I would work with your SFDC Admin to hide records from the Marketo Sync User when the "Receive Marketing Materials" field is checked. This will prevent the lead/contact from syncing back down to Marketo.
An overview of SFDC sharing rules can be found here: https://help.salesforce.com/HTViewHelpDoc?id=security_about_sharing_rules.htm&language=en_US
Best,
Will