As long as you canceled before the free trial ran out, you should be okay. Otherwise the instant the trial ran out your paid subscription would go into force and you would be charged.
Did you take out your free trial through Roku, or directly with CBS All Access? If through Roku, log into your account at https://my.roku.com/account/subscriptions and make sure the subscription is not shown there as active. If directly with CBS All Access, do the same on their website.
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