Welcome to the Roku Community and thanks for posting regarding your inquiry of accessing the Roku Smart Home mobile app.
Unfortunately there is no way to log out of the Roku Smart Home mobile app remotely. The only option to ensure that a remote mobile app does not have access to the Roku Smart Home cameras would be to create a new Roku account and factory resetting the Roku Smart Home devices and setting them back up on the new account.
Hope this provides insight into your inquiry. If you have any inquiries in the future, please don't hesitate in posting again in the Roku Community.
Roku Community Moderator