If you are being billed by Roku, any such subscriptions will be listed on your Roku user account page (https://my.roku.com). Check there first. If there's something listed there, you can cancel it right there.
If you find no subscriptions there, then you are not being billed by Roku, regardless of what your credit card statement says. That information can be falsified by people using stolen credit card info to trick you into paying a bogus charge. If there is no subscription on your Roku account, you need to contest the charges with your CC company and probably have your card cancelled and replaced.
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