Unscheduling a blast will prevent a scheduled blast from sending, and put the blast back in Draft mode for you to send at a future time.
To Unschedule a blast, locate it on the Blasts page. Click the carrot next to the Edit button.
IMPORTANT - Selecting Delete will remove the Blast from the blasts page, but not prevent it from going out. Always select Unschedule if you don't want a blast to send.
Click Unschedule in the confirmation window.
Wait for the message to appear confirming your blasts was successfully unscheduled.
Your campaign will revert to a DRAFT on the blast page, for you to either Delete or Edit and re-send at a later time.