I mean a lot of this would depend on the situation. You could basically try to get their IP and maybe put it under the hashtag #fediblock, maybe other admins will block it, but seems unlikely. It would be nice if we could have a regex based block function, but we don’t currently. You could change your username and picture to make it harder to find you, but I don’t know what other tools you might have.
This is why I favor Matrix, I don’t have to give anybody any info and it would be hard for the government to build a social graph of my contacts if we use VPN’s or Tor to connect.