New newsgroups will appear in the New Newsgroups subscription. You can delete that subscription at any time. A new one will be made the next time that new groups appear. To keep track of what is really new, look at the groups in the New Newsgroups subscription. If any are of interest to you, drag them into one of your own subscriptions. Then either delete the New Newsgroups subscription or clear it with the command from the Subscriptions menu. If you delete it, it will be recreated the next time that subscription appears. With either scheme, the subscription will have only the groups that are new since you last looked at it.
Note: New groups are automatically added to the All Newsgroups subscription and any other automatic subscriptions (those with bold text names). You do not have to drag them there.