Thanks for following up. To help clarify the answer here, autoplay functionality is determined within each channel, by the channel developer. This is not a Roku system feature as of now.
Most channels (apps) that offer this feature provide the ability to enable or disable it within their individual 'Settings' menu inside the channel. I'd recommend following up with any specific channel provider for more information and assistance with adjusting any setting within their channel.
More specifically it would be nice if Roku offered the ability to toggle AutoPlay ON or OFF on the content that you they are offering. I don't subscribe to any 'Channels' thru Roku, but I do stream ad-supported content that Roku makes available thru The Roku Channel. There currently is no ability to turn OFF Autoplay. It can be frustrating to begin watching Episode 4 of a series at night and if you doze off you may wake up and now you are half-way through streaming Episode 6. With a Roku TV I have learned my lesson and use the SLEEP TIMER feature and set it to 1 hour when I begin watching a 42 minute episode at night. Worse case then is I may go 18 minutes into the next episode.