Once you have closed and collected all the signups for a whitelist, it is time to remove the collected data, for your own security (to not access the wrong data next time around) as well as for the privacy of your user (so you don't have access to their addresses longer than needed). To do so you can simply run /configure-whitelist remove. Once you have run the command, select the whitelist you want to delete and all data associated will be gone.
This action is destructive and IRREVERSIBLE. You will have no further access to this specific whitelist's data once it is removed. If you just want to stop signups temporarily our permanently, without deleting all signup data, use /configure-whitelist close instead.