If you have lists that you no longer need, you can delete the ones that you own and/or leave Shared lists that another user invited you to join.
On iPhone & iPad
To delete a list on your iOS device, swipe from right to left across the list you wish to delete. Tap on the red trashcan icon, which will bring up a confirmation prompt; confirm your action by selecting “Delete List” to delete your list.
To delete a list on Android, tap on the list you wish to delete to bring up Item view. Then tap the three dots in the top right corner and choose "Edit List." You will then be brought to the Edit List window, with the option to delete the list at the bottom.
Select the list you wish to delete from the List view and simply press Command + Delete/Backspace. Alternatively, right-click on the list and click “Delete List,” or "Leave List" for Shared lists.
On the Web
Select the list from the List view and simply press Ctrl + Backspace. Alternatively, double-click on the list you wish to delete, and then click on the trash can icon located in the lower-left hand corner of the Edit List window, or right-click on the list's title and choose Delete List (or "Remove List", if shared) from the context menu.
On Windows 10:
To delete or leave a list, right-click the list and choose "Delete list" or "Leave list" from the context menu.