If it works anything like Mastodon, server instances may ban entire other instances from posting and interacting with their community. So you can't just create a terrorism instance and expect to be able to spam every other instance. For accounts on each instance, the admins will have to ban them just as any other account.

There's more than a few that have just been mass defederated by most instances

Same as most places. Moderation, ban hammer, and for Lemmy specific solution, defederation with a problem instance.

Hexbear and lemmy.ml both had their share of brigading and trolling to deal with. It will likely become worse as it grows more popular.

The modlog is a useful tool, though it doesn't capture everything

making meme’s obviously

the reality is, there will end up a pocket group with this and other types of content, over time it will be effectively a shadow network. the blocking game will always be a thing.