Unsyncing certain leads between Marketo and Salesforce

It is possible to do this and there has been a previous thread on how to do it.  But. . . it creates a whole slew of other issues that you have to deal with that in my opinion really are not worth it.  After you create the custom check box "Sync to Marketo" and only if it is true do you sync with Marketo, now you have to deal with all the scenarios of when you would toggle that on and off. 

Is it possible to "hide" leads that are in Salesforce from Marketo?​

This is also a guarantee way that you will create duplicates.  If you have a "dead lead"  and they live in SFDC but not in Marketo, and they come back around and fill out a form, attend a webinar, anything that would put them in Marketo, they are now created as a new lead in Marketo and a duplicate in SFDC.  If a sales person emails someone in SFDC that is dead and now they respond to an email, would you want to know market to them?  But they are not in Marketo.  There are easily dozens of scenarios that you would have to account for and write custom rules in Marketo and SFDC to account for. 

In my opinion it has never been worth it, if they are truly dead I have seen folks archive the record and remove them from both Marketo and SFDC.  It is much easier to have a 1:1 relationship with Marketo and SFDC.  Also, when you delete the record from Marketo you will lose the historical data as well, program membership, activity history, etc.  The amount of money that you save could be less than the effort and time you put in managing two databases.