Membership on a list can be modified in bulk. Start by Logging In to the online interface, then click Review Members
Delete in Bulk
Finally, click Delete selected email addresses
This will remove all the members from the list on that page. If your list has more members, the screen will reload and show the next set of members.
To speed up the process, set the page size to 500
Repeat until you get a message that the list has no members.