The fastest way is to message us here in chat. Tell us you'd like to cancel and we'll take care of it immediately — no forms and no waiting.
You can also cancel yourself through the Stripe customer portal. The link to it is in any billing email Stripe has sent you when you were charged.
Cancelling inside the Publicus app isn't something you can do yet. We're moving billing into the app, but we don't have a date to share.