Got a citation on this? I'd love for it to be true, since it'll save me a lot of effort. But WSJ, for instance, requires you to cancel via phone even if you sign up online.
Nice find. From some other quick research, if there is a violation, you have to report it to Visa via phone or mail.
Looks like they used to have an online form a several years ago, but it went missing. Various blog/forum posts indicate that most people have had no luck getting any sort of response from Visa via mail.
Section: 5.9.8.2
"Provide an online cancellation procedure if the Cardholder's request for goods or services was initially accepted online"