You know how to tag your subscribers, but what about the Subscribers that don't have any Tags? Here's how to find them:
First, Create a New Segment:
Select 'All Subscribers' as your first Filter Group:
Add a new Filter Group matching [none] of the following.
Add a new filter for every Tag in your account.
Voila! Anyone left in this Segment doesn't have any Tags!
NOTE: Since ConvertKit segments always target your subscribers, technically, the first part of this segment with the "All Subscribers" filter isn't necessary, but we often include it to help us remember exactly what the segment is doing.