I don’t think Lemmy proper has that feature yet, but some apps might.

Other than that you could open an account on an instance more in line with your prefered blocks or use kbin which already allows for instance blocking.

From the twitch pages

The desktop web browser experience of twitch.tv and supporting sites, officially support the latest two versions of Google Chrome, the latest two versions of Mozilla Firefox, the most recent version of Microsoft Edge, and the most recent version of Apple Safari.

https://help.twitch.tv/s/article/supported-browsers?language=en_US

What’s your browser version OP, so we can rule out whether it’s just a version issue