When the lead/contact is deleted from SFDC, in Marketo it will set the field "SFDC Is Deleted" to true. If you want to automatically delete those leads from Marketo as well, you can create a smart campaign that deletes the lead in Marketo. The smart campaign can be either a trigger campaign (tigger is Data Value Change, SFDC is Deleted, new value true) or a batch campaign (filter is SFDC is Deleted is true) that runs daily/weekly/monthly to clean-up the database.
Regards,
Sheila