Deleting Leads From Marketo (and Not SFDC)

Hi there!

We are working on some cleaning up of our Marketo database and we found several hundred leads that do not have email address listed. I'd like to delete them out of Marketo (and not Salesforce). By doing this, will they come back into Marketo during the next sync period?

What is the best way to make sure leads without email addresses will not come into Marketo.

Thanks!