If the question is concerning deleting messages from the POP Peeper inbox then as always you can optionally select as many messages as you want that are listed in the POP Peeper inbox using the CTRL and/or Shift keys to select the messages to be deleted and subsequently clicking on the Delete button (CTRL key to select non-contiguous messages as a group and/or Shift key for a contiguous group of messages).
Actually this also applies to removing messages from the POP Peeper inbox as well (note: messages will return on the next Mail Check if the messages still reside on the Email Clients Server) by right-clicking on selected messages in the POP Peeper inbox and then selecting "remove from list" from the context menu.
Good judgment comes from experience and a lot of that comes from bad judgment. - Will Rogers