We have a vast amount of "dead" leads in SFDC which we want to keep in there but remove from Marketo. We have already synced the two systems, is it possible to essentially hide these leads from Marketo?
I agree with Grant on 1:1 syncing. However, there are also valid reasons to keep some records out of Marketo, for example when a lot of them have no email address. In addition to using SFDC Sharing rules (only possible with SFDC Enterprise Edition and with a private SFDC sharing model), you can also use a Custom Sync filter. Create a check box field "Sync to Marketo" and contact Marketo Technical Support to enable that as a filter. Then when you check the box, it will sync.