You can slow your sync down to once every 30-35 minutes, but I believe that is the slowest you can go. One idea is that you can create a smart campaign that monitors Leads synced from SFDC. If the Lead does not have a box checked (or certain Lead Status, or however you would like to denote that the Lead has been reviewed and approved), then Marketo will remove the Lead from Mkto, but remain in SFDC. Then, you would need aother campaign that would look for that same checkbox, status, etc. so that when the Lead is approved you can pull it into Marketo.
Not ideal or sophisticated, but I think that should help you manage your data in the manner you want.