I dealt with Virgin's debt collectors twice and they were reasonable and actually offered better customer service than Virgin itself. When I explained them the situation and why I cancelled the payment they immediately dropped the case and I haven't heard of anything since (it's been 2 years since the first case).

Of course, you should always aim to cancel properly, but if the company is intentionally being obtuse and you've made reasonable attempts to cancel "within the rules" then just cancelling the payment is a valid option.