Call off a night
One moment…
Sign in on the team page and come back.
This stops the website selling tickets for that date within seconds, puts a "this one has been called off" notice on the event page, and stops the day-before reminder going out.
With both boxes ticked it also refunds everyone who paid and emails them that it's off. The email goes first, so nobody's first news of it is a bare "Refunded" receipt. Martin gets told either way. A refund can't be undone, so if you're unsure whether it's really off, ask him first rather than pressing this and putting it back.
Everyone who paid, and everyone who pressed “I’m coming” on the site for this date. People who just turn up, and anyone who signed up on Meetup, are still yours to tell.