All part of Virgin Media's long running failures to enable customers to easily cancel their services without hassle. This is a flagrant breach of Ofcom Condition C1, and indeed the Fairness Commitments that VM put their name to a few short years back, but apparently nobody cares. Especially nobody at VM. Do not cancel the direct debit - VM won't contact you, won't react, won't notice at any human level, but their automated systems will, and they will mark your credit history with a default (a stained credit history lingers for six years and can affect loans, credit cards, mortgages, energy supply, telecoms, car plans, even employment checks my throw up the default). And VM will add late payment charges and farm it out to a debt collection outfit who will come knocking.
What happens next depends on how strongly you feel about being treated with utter contempt.
1) Easy option, wait for the forum staff, they can probably sort this out, given that you've already told VM that you want to cancel (hint to staff this is the umpteenth report like this, "we don't process cancellations in the forum" is the wrong response, notice has already been served, but not processed). But see comments below - just backdating a cancellation they've failed to process for five odd months is not on its own enough to settle this.
2) Raise a formal written complaint (if you haven't already, donkey post, recorded delivery to the same Sunderland* office as cancellations go to), see if it is handled well. Many are not. Make sure you get (a) the account cancellation backdated to the original date it would have been cancelled if VM's agents had been competent, (b) all subsequent charges refunded, (c) compensation of say at least £120 for the distress, inconvenience and protracted failings here. If the company's offer is short on any of those (that includes the forum staff actions to resolve this even if you haven't formally complained), then you reject any "resolution" the company offer and ask for a deadlock letter, and then escalate to CISAS, asking for the contract to be cancelled, for this to be backdated to your first attempt to contact the company, plus additional compensation any failure beyond today's date in resolving this.
* I'm assuming your cancellation got sent to Sunderland? For a couple of years up until February of this year VM's web site and online documents also referred to a Nottingham address that they hadn't actually used for consumer services for the previous two years. If that's the case, the advice above still applies, because VM should have sorted out their grotty, poorly maintained website and/or written to every customer telling them that the address for postal cancellations had changed (or simply continued to use post forwarding like normal, decently run companies would).
