Several things you could do as an admin:
- Switch from open, to allowlist or blocklist federation mode.
- Make sure you have captchas on your instance.
- Ban manipulated accounts.
- Remove the manipulated posts / comments.
- Make sure you have a low signup rate limit.
- Close signups.
An instance could maybe protect its users against this by blocking the domains of evil instances, but does this approach scale?
Yes it does, people will naturally share / sticky blocklist posts of malicious instances. This happened organically with mastodon and the #fediblock tag, and it will likely happen with lemmy as it grows.