You might not see a fix for this behavior any time soon. Part of the Chromecast's appeal is its easy configuration, and altering the disconnection response might create more headaches than it solves. As it stands, you won't have much reason to worry. Even if pranksters build gadgets like the Rickmote, they'd still have to get within range of your Chromecast's WiFi -- if someone tries to rickroll you through your TV, you'll probably catch them in the act.