Don't use the OK button on the remote to dismiss the screensaver; use any other button. One or more of the Roku screensavers advertises channels and pressing OK will install the channel. I don't believe that any of those channels cost anything to install. Also, enable a PIN on your account at https://my.roku.com.
The problem only happens when I have paused a show. While the channel installs, it disconnects the tv show I am watching. No one else is even in the house, so I know it's not being added by another user.