Thanks for the info!
If you go to larian.com and log in, you can go to the Account Menu. There's a button on the left middle of the screen that says "Connected Accounts." Click on that and then click on the Twitch box. It will prompt you to enter your Twitch user name and password. Once you do, it should display "Disconnect" at the bottom of the Twitch box, which will indicate that the account is connected and you can use that link to disconnect them.
If that's not clear, there's an FAQ at
https://baldursgate3.game/news/announcing-baldur-s-gate-3-twitch-drops_74. The "sign in to your Larian account here" link on that page takes you directly to the Connected Accounts screen.
