Open History > Scheduled (future messages) or History > Sending (due now). Tick the rows and use the delete action ("Are you sure you want to delete N messages?"), or click Cancel All to remove every message in that tab. You can also open a single message by its ID and click Cancel Message. Cancelled messages are removed from the queue and never charged. A broadcast's remaining queue is also visible on its Group Messages detail page under In Queue. Messages already in the Sent tab cannot be recalled.