I am a bit new to this plugin and I got the same kind of issue:
First, I sent a newsletter to nearly 2000 subscribers. Two days later, I got another batch of 500 new subscribers. I needed to send them the same newsletter without resending the newsletter to the first 2000 subscribers.
I waited few days to let the first recipients to eventually unsubscribe or else. After that, I backed up the table wp-newsletter from the data base. I exported the 2000 subscribers in a file. Then I deleted the 2000 subscribers. The subscribers list was empty. I imported the second batch of 500 subscribers. I did the second newsletter sending. Then, I imported the 2000 subscribers.
A bit complicated procedure. If you know better, thanks to share 🙂