Hi Margaret,
Once you've hit send, the messages go out, and you can't really recall them. Pending is a status which I believe refers to messages that are still attempting to deliver. They've already been sent by Marketo, but they haven't yet been received by the recipient's mailbox (but they haven't been marked as bounced yet). There's some good dialogue about this here:
'Pending' Emails?The "stop date" function works to limit future runs of the campaign itself. So, for example, if you had a recurring campaign going out each week, setting a stop date would prevent that from recurring after a certain date. It can't, however, stop a send or recall messages after they've been initiated though.
Usually emails in a "pending" status are only there for a few hours at most. If you're seeing a longer lag time than that, you might want to submit a ticket to support, just to double check everything on the deliverability front is functioning.
Hope this info helps!