If you don't already have the private sharing model implemented in Salesforce, implementing it may have a significant impact on your organization as users (including the Marketo Synch user) will be able to access only the data they own. No one will be able to view records owned by others unless permissions are made available for non-owners of records using the Role Hierarchy, Group Sharing, etc. Record owners can grant access for others to specific records they own for otherwise unavailable actions (read, write, transfer).
Since this is a one-time exception, it may not be worth the effort / disruption. Leaving the email address field empty will accomplish this as a one-off work around.