If both your Marketo and SFDC databases are completely synced, every lead created in Marketo will also be created in SFDC. If you already control which leads get synced to SFDC based on certain criteria, you will just need to create a trigger campaign to direct these leads to a static list or designate a lead source that's not synced to SFDC.
If the first scenario is the case, you can still add people who fill out the form to a static list, export it, then delete from Marketo/SFDC. It all depends on your sync setup.
Hope that helps!
JK