You control the sync between Marketo to CRM (and you can even do this between CRM and Marketo using the custom sync filter). Out of the box, leads don't automatically get synced to CRM from Marketo.
If you have a global sync rule in place that automatically syncs all new leads, then that would need to be dealt with. I would recommend if your usual practice is to sync everything, and in this case you don't want these leads to go into SFDC, there are a small number of them, etc. I might bypass Marketo all together.
The other work around is that there is a minimum number of fields that are required for the SFDC sync. Last name and Company Name.
If you brought in these leads with just their emails, and you did not append a default of <unknown> when there is no Last Name and Company name, then those leads will fail the SFDC sync and you are good.