Unfortunately no, its not bannable and its to the responsibility of the streamer to hide their map. There's also no way to prove somebody is ghosting, as they could have a different twitch name from their league name in the chat and riot wouldn't have the information that you were watching twitch while playing either. You can link your twitch and your account, but most players in high elo have multiple accounts they swap between dodge timers anyway.
I really wish it was, I play with and against streamers all the time and typically at least somebody is ghosting them each game. Infact its so common streamers accuse players for ghosting if the enemy simply invades them. I've been accused of stream sniping for invades against players who I didn't even know were streamers due to being on alt accounts, just because I know how to track players on the map. Can you blame them? No.