Sadly I don’t think it’s possible on the Lemmy-ui/server level, but some mobile apps allow you to (maybe just one, note sure. Either Memmy or Voyager since I use them interchangeably). I also look forward to the ability to block instances fully on desktop.
I don’t believe that there’s a lemmy-ui or server side option for it, but some mobile apps do let you block instances which for me is good enough.