When blocking communities, they will not show up in your “all” feed anymore. This makes browsing Lemmy nicer especially now that you might not subscribed to a lot of communities.
In the browser: In you Lemmy instance, click on your username (right upper corner), “Settings” and open the tab “Blocks” and type in the community you would like to block. Here you can also unblock communities
On Jerboa (Android): Open the community you would like to block, click on the 3 dotted menu (right upper corner) and click “Block Community”. Note: you can not unblock on Jerboa. Use the browser (Settings -> Blocks) to unblock.
Now we just need an option to block an entire instance. Not that I needed it so far, but think it would be good to have that option.
I’m on kbin and there is a way to do this on the desktop site at least. If I click on the domain it goes to a page viewing it with a block button. Maybe what you’re using has something similar?