Select a volume or range of volumes (shift + click and or control + click) from the inventory window to remove from container/s. Volumes assigned to different containers can be selected and removed at the same time, there is no need to select one container at a time.

From the Container Selection window check the Select No Container checkbox at the top left. This will remove the displayed containers.

Click OK to remove the volumes from the assigned containers and exit back to the inventory window.

Select required container/s and right click, select Special Operations → Container → Purge

Then click Yes to Confirm purge popup.