Cancel adobe stock without cancellation fee

Contact Adobe customer care. It is indeed such that the terms you agreed upon state that if you do not cancel, the subscription will continue on a paid basis as an annual contract. I would strongly recommend to check in the future your finances at a regularly so that you do not incur unexpected charges. Three months is (in my eyes) a tactic agreement on the subscription. You can even not argue that you thought your subscription had been canceled.

You accepted the terms of the Free Trial, which required you to cancel within the first 30 days, after which the Free Trial converts automatically to an Annual Subscription billed monthly.  
Adobe Stock free trial - Get 10 Stock images free. | Adobe Stock | Adobe Stock
The fact that you didn't notice the credit card charge for 3 months does not put you in a favorable position; however, if you haven't actually licensed any assets during the subscription, you can contact Adobe and try to negotiate a waiver of the cancellation fees. The Stock Free Trial should have converted into a $29.99/ month subscription, so the $80 charge you're seeing must be for something else as well. Log on to your Adobe account to determine which other subscriptions are now active.

Adobe customer care can be contacted  at https://helpx.adobe.com/contact.html?rghtup=autoOpen. Pop-up blockers need to be disabled and you'll need to accept cookies.