We mark all of our customers as "Is Customer" in Marketo and then have suppression lists that prevent email matches from getting our lead/prospect emails.
Typically "acquired" lists have a domain column (or one can be created from the email address column) which is great for matching. You can create a filter on your campaign/or a suppression list that would compare your list of customer domains (will require manual updating of the list) and remove all leads with a matching domain.
This is not exactly what you're looking for but should help you get a little closer?
- Jeff