To delete subscribers, go to the 'Subscribers' page from the main Mailing Lists menu. Below the tools used to add subscribers, you will see a list of the current subscribers.
In the right-most column there is a 'Delete' link for every user. Click the link, then click 'OK' in the confirmation box.
There is no method to bulk-delete users. Deleted users can be re-added in the 'add subscribers' section.