Thanks for the question - this is a feature that is currently being worked on and should hopefully be available in the next general release for any lemmy instances running 0.19.0 and above. The main delay here was waiting for lemmy to natively support blocking instances.

If you'd like to keep track of the on-going progress for this feature (and general support for lemmy 0.19.x), see this GitHub issue!

Right now, this feature is client specific. Some have it ( like Sync ), most dont.

The happy news is , a serverside option to block an instance is already merged and will be available in next (big) Lemmy upgrade 19.0. After that, the client apps will easily be able to implement it ( Now, i guess they are just filtering posts client side if from that instance )

Click on the instance icon on a post, it'll take you to the instance page. Across the top will be several options. Scroll over to "block" and click that. It seems to take effect the next time you open or refresh Lemmee.

Edit: I should add, I have Sync, not Thunder, but it might work in a similar way.