I am sorry to hear about your experience with customer support and will address this with the teams involved.
To cancel an annual membership you do need to contact support as documented here - Cancel your Stock membership
I can see that you signed up for a Stock subscription on 8th October and received a refund for the first month as part of the promotional offering.
The plan then renewed on 8th November and 8th December - £23.99 for each month.
As you have not used the image allocations for these months I will arrange a refund as a gesture of goodwill.
This will mean that all 3 charges associated with the Stock subscription will be refunded.
I had a very similar experience.
I also cancelled during my Stock trial, but the charges picked back up months later and now your customer service (case 0221150572) informs me since I am outside the trial I must pay a penalty to cancel?
Why is cancelling so tricky? Why the lapse in charging people? Is this to trick the consumer? It has a very bad feel to it.
Looking at your account, you signed up for the Stock subscription in September and mentioned you'd cancelled in Nov/Dec, which was outwith the trial period so a cancellation fee would apply. Instead, it seems you accepted an offer of 2 free months to continue with your subscription which is why there was no charge for November and December then billing resumed in January.
Your plan is now cancelled and all credits have been used.