You might contact the Citizens Advice in UK who help in cases of such issues. You might look at Cancelling a service you’ve arranged :
Contact the Citizens Advice consumer helpline on 0808 223 1133 if you need more help - a trained adviser can give you advice over the phone. You can also use an online form.
If you’re in Northern Ireland, contact Consumerline.
An adviser at your nearest Citizens Advice will also be able to help you argue your case or argue for you.
Generally, you have a contract with this business, and if you just stop paying they might keep delivering their goods and send a debt collector. Usually a letter with a written cancellation is always strongly recommended.
Here they argue (for UK) that e.g. a letter of cancellation might be sufficient, see below. On the above referenced Citizens Advice webpage they have some templates for cancellation letters.
There is no specific requirement as to how notification to cancel a contract should be given but in the event of a dispute the burden falls upon the consumer to prove that he did cancel within the cancellation period. So, for example, if he rings you but calls the wrong number, he will have failed to cancel.
As a trader, you have an obligation to provide information about how to cancel as well as the means to do so (a form in a prescribed format that your customer can use to give you notice).
But whilst you must provide the model cancellation form in your system or process, the consumer is not obliged to use it. He can notify you in any way he likes, and in hard or soft copy. That includes, by letter or written note, by e-mail, or even by an SMS text message.
If you offer an online cancellation form on your website then your customer may use that, but if he does, then you must acknowledge receipt of that notice without delay through what the Regulations call a durable medium - e-mail message, written letter, or text message.
Of course, the cancellation is effective only if the communication is sent to you before the end of the cancellation period.
To emphasise, the key time is when the communication was sent, not when you received it.