This has now gone live. 🎉 We've added a 'delete customers...' option that can be selected when you are viewing a specific segment. This option will delete all the customers currently in the segment. This means that only the customers who are listed in the segment from the last time it was refreshed will be deleted.
There is a very strong warning displayed when performing this task as there is no way to retrieve customer data once they have been deleted. Here's a screenshot of where you can find the option: