I had a similar experience with GoDaddy. The magic trick was to put it in writing. An email was sufficient, make sure you are explicit - "Thank you for my free trial that commenced on xx/xx/xxxx. I do not wish to proceed. Please cancel my account"

Then, when / if they reply with whatever upsell they offer, I replied with - "I sent you this email on xx/xx/xxxx. Again I repeat, I want to cancel my account. If you charge me, I will raise this with my bank as an invalid transaction"

When I got charged, I printed off the emails (there were 3 incidents, 12 - 15 pages per incident) and I emailed this to the bank. Like magic! The bank reversed the transactions.

Keep the emails somewhere safe. If a collections agent were to call, I would supply them with the 12 - 15 pages.

Remember, when you initiate the chargeback, it is the vendors responsibility to prove you initiated the purchase. Not the other way around.