click on the OutQueue entry in the accounts list. This will show you a list of all the messages waiting to be sent by VPOP3
Select the messages you want to delete, and either Hold or Delete them. Holding a message will leave it in the OutQueue, but VPOP3 will not try to send it. Deleting a message will remove it from the OutQueue permanently.
Note that you cannot delete items from the VPOP3 OutQueue whilst VPOP3 is in the process of sending them, so you should stop VPOP3 from sending first.