The proper procedure to cancel a VIP subscription is:

1.)  Contact the support using the in-game link "Contact us" at the bottom of the website page.

2.) Title your message "Urgent Cancel Subscription" and provide the account name you want to cancel the subscription for.  (Because each account is given a special link, you should send one cancellation request for each account that needs the VIP subscription cancelled for.)

3.) Within a few days you should recieve a response with a special link.  Click the link and follow any other instructions you are given.