Defederation actually does work both ways if the instance enables AUTHORIZED_FETCH. That setting requires 3rd party systems to prove their identity before they can retrieve any data, which allows an instance to block defederated domains. I don’t know if Lemmy or Kbin supports that, but practically all of the microblogging fedi software does (that being Mastodon / GlitchSoc, Pleroma / Akkoma, Misskey / FoundKey / FireFish, and GoToSocial).