I don't see an easy way to unsubscribe everyone at once in a mailing list. I need to keep the list in sync with a list in my database, but I have a bunch of custom fields, and if I delete the whole subscriber list, I have to set up all those custom fields again, which is a pain. It seems like it should be an easy thing to leave the mailing list in place and just remove all the names at once so I can upload an up-to-date list.
We're looking into an esier way to do this shortly, but in the mean time there's one approach you might consider.
When you re-upload the list, we'll update any existing subscribers with the new details in your file. If there are additional subscribers that shouldn't be in the list any more, you can always add them to the suppression list and they will be automatically removed from the list.