Leads sync from SFDC to Marketo by default, but they don't have to automatically sync the other way, from Marketo to SFDC.
If your Marketo database always syncs leads to SFDC automatically, my first inclination would be to say that you've got an active trigger somewhere that says "Lead is Created --> Sync Lead to SFDC".
If you want to prevent the automatic 2-way sync, first and foremost you'll need to turn that trigger off, but then you'll need to remember to sync leads to SFDC on a campaign or ad hoc basis going forward.