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IF YOU SEE "MANAGE SUBSCRIPTION" INSTEAD OF "REMOVE CHANNEL":
A channel whose subscription was made THROUGH ROKU can not be removed via the onscreen user interface while the subscription is active. The "Manage subscription" option will allow you to prevent any additional payments and keep the app from auto-renewing when the current pre-paid subscription runs out, but until then the subscription remains active. The "Remove channel" option will return when the subscription period expires.
As a work around, for immediate channel removal you can use the official Roku remote app on an Android/iOS phone/tablet. This will only remove the channel from your Roku; it will NOT cancel the paid subscription, but it will then allow you to re-install it.
• At the bottom of the first screen, click "Devices", and if necessary connect to one of your Roku devices.
• In your connected device click on "Apps on Roku".
• Click AND HOLD the channel you wish to remove (a quick click will bring up the remote controls instead).
• Click on "Remove" and confirm.