Open the campaign and click the red Cancel Campaign button in the header. The campaign is marked Cancelled (a badge appears next to its name) and every message still waiting in the queue is marked cancelled so it will not be sent.

The button is only shown while the campaign is not yet cancelled and its scheduled send time is still in the future. Once the send time has passed the button disappears; if you somehow submit a cancel for a campaign that already started you get Campaign already sent!, and for one already cancelled Campaign already canceled!. Cancelled campaigns cannot be edited or re-activated – create a new one.