Kate-
With the caveat of "
be super super careful whenever dealing with rules that delete leads" I'll offer up the following:
Yes, it is possible to delete contacts without email addresses. Simple create a new smart campaign with "Email address is Empty" for the criteria. Then, for the flow, select (from the "Special" menu) "Delete Lead" and if you are syncing to a CRM like SF, also include the step "Delete from SFDC". Create a schedule of when you want it to run, and you're in business.
Be aware, doing this will discard all behavior data associated with a lead. So, for example, if you don't have a lead's email address right now so you delete the lead, then 3 days later they fill out a form on your website, you essentially lost all of the leads web browsing activity history from the past 90 days.
I'd highly recommend considering, at minimum, a smart list filter that also checks for "not visited webpage: any; for: 90 days" (or something like that) to avoid deleting any leads that have associated browsing activity.
With respect to the duplicates question, your best bet there is finding another tool that specializes in dupe management. We use Cloudingo here, but there are many good product. Lead merging is a very complex process that involves finding matches in company name/email/ first-last/ etc. using some kind of loose string matching. Then once a match is found, setting up logic that identifies conflicting data merging priority and behavior so that you are not accidentally over-writing the wrong data is essential. Marketo does some bare-bones deduping (for example, when you upload a list of leads where some of the leads are already in your Marketo instance) but its not designed to be a full on dupe-manager.
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