Hi,
You can use the new segment feature that is available since version 6.17.0. When sending your mail on the tab where you select your lists (in your case Last A) you can scroll down and indicate that you want to use a segment. Select Yes and click on Save & Continue. On the next tab you can define your segment. In your case users that are not on your list B.
You also can predefine your segments in case you frequently need the same segment: https://docs.acymailing.com/main-pages/segment.
Alternativly you could use a mass action availiable under the menu item Automations.
