The best way to achieve this would be to edit the entry rules of this series to be something none of your users will match. An example of this would be to create a new tag and changing this to be the entry criteria 'tag is __'. Therefore nobody will match it as the tag hasn't been added to anyone.

With this new entry rule in place, the customers who are already within the Series will continue through it, but no new customers will be able to enter 🙌