You can cancel a mailing within 5-15 minutes of clicking "Start Mailers."
After that, the mail will be printed and dropped in the mail. Once that happens, just like dropping mail into the post office mail box, you can't reach back in and get it out!
If you're within the 5-15 minute window, open the deal and click the three dots in the top right.
Then click “undo”. If “undo” is not an option, just trash the deal and it won’t send. If it has already been more than 15 minutes... it’s already sent.