Hey guys,
The last step here is that you need to have Marketo create a custom sync between SFDC and Marketo using a particular field. We used the boolean field 'Hide From Marketo.'
When 'Hide From Marketo' = True, then Salesforce and Marketo cannot sync this record (in either direction).
The default 'Hide From Marketo' = False, allows for full sync.
So if you need to delete from Marketo, but keep it in Salesforce - mark the field 'Hide From Marketo' = TRUE, wait until it syncs to SFDC (I usually wait 24 hours) and then delete from Marketo (delete from CRM = False).
Good luck!