Archiving lists (Deleting lists)
Lists cannot be permanently deleted. Instead, they can be archived to move them off of a board.
Archive a list by selecting "Archive this List" from the list menu that appears to the right of the title on hover. (It's not currently possible to permanently delete a list.)
You can archive a list when you are done with it.
Reopen a list by selecting "Archived Items" under "More" within the board menu. Then click "Switch to Lists" near the top of the page, then choose a list and click "Send to Board". (It may already say "Archived Lists" at the top, in which case you won't need to click "Switch to Lists.")