So, to do this, you’d want to create a tag for “Active in Campaign”, you can do this under CRM>>Settings.
Then you’d need to apply this tag to anyone active in a campaign. You can use the “Unique Campaign Contacts” report, under Marketing >> Reports. (Note: If contacts are active in multiple campaigns, they will show up twice on this report, that’s okay.)
Once you’ve run that report, with no criteria, go ahead and click Actions, then apply/remove tag. Apply the “Active in Campaign” tag.
Now go to Marketing >> Email Broadcasts, and build your broadcast, when you get to selecting recipients you can click “New Search”, and then choose anyone who does NOT have the tag in question. That’s your audience (though you may want to remove hard bounced email addresses, unsubscribes, or people who haven’t engaged in a few months (You can find those groups using Email Status Search, also under Marketing >> Reports).
Hope this helps!