You can still use the apps until the current subscription period expires. Cancelling only cancels the auto-renew of the subscription. Once the current period expires, the "Manage subscription" option will be replaced with "Remove app".
To add to the response from renojim :
A channel whose subscription was made THROUGH ROKU can not be removed via the Roku onscreen user interface while the subscription is still in force.
As a work around, for immediate channel removal from your Roku unit 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.
• 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.
