Did some testing with a private channel and test user. The in-channel subscription indeed appears in the list of subscriptions for a user in the user's Roku control panel, and there's an option to cancel the subscription.
By "Roku control panel" (?!) do you mean the web, https://my.roku.com/account/subscriptions ?
I myself was wondering about the player itself - when you press the remote Options button [*] while over the installed channel in home screen, in the local menu that pops up there would be "cancel subscription" - and since potentially there could be >1 per channel, what happens if that is chosen?