I’m a Jamf School user and can tell you how I do that same thing in Jamf School. I can’t talk to Jamf Pro.

Go to Users > Users > click the down caret in the upper right to bring up the Filters. Select ‘Has Device’ = No; and ‘Apple School Manager User Role’ = None. This will select all of your users that do not have an iPad assigned and that do not have an Apple Account. If you don’t use Apple Accounts, then you can ignore that part. I always make sure to ‘Move to Trash’ in case I need to revive them in the future. You can always clear them out of the trash later.

Screen clip included:
 

I hope this helps.

I would get a CSV of all users and run a script to compare to users in Jamf...Pretty straight forward. You can probably use this API and see if each one is there. If not, let you know.https://developer.jamf.com/jamf-pro/reference/findusers

I get a CSV of all users from Jamf Pro, and sort it by number of devices. Anything that comes back with all zeros gets deleted. To mass delete them, I use The Mutt (a great tool) to upload the users to a static group and from the I send the Delete Mass action command.