It's not just memes but also unrelated content to the magazine. There are spammers out there, hunting for quantity.
The short answer is probably: you can't. You say you don't want to block the mag, but how would you make the difference between useful posts and spam from the same mag? You can't, it's just too much content to sort out.
The only reason why we did not defederate was a lack of content. Once kbin.social is filled with enough users federation won't be necessary anymore and it will be seen as a way to introduce cheap content to prime newborn instances.
Can you tell the difference between lemmy.ml, beesomething and lemmyworld or the cohort of attempts at lemmysomehing? No. There isn't much difference. The idea was that instances would specialize into a theme but it did not happen. People want the mass. They want to be where the buzz happens.
Yes, in theory you can use any instance as a point of entry but in reality people want to join the biggest party. So the only difference between instances is mostly moderation. And somehow the initial link that people saw in their original community to find their new instance. Like many of us did with /r/redditmigration
This is all about sane default settings. If your instance allows everything and let you chose what you don't want to see then you are trapped into a whack-a-mole game against a million moles. You will lose your mind blocking stuff constantly.
Either
we defederate spammy instances, like you mentioned lemmy.world
we block a mag, and other mags from the same instance will popup anyway.
People won't have the patience to block the meme spammers. In short: the blacklist system we are using no cannot handle the increasing load of meh content, we have to use the whitelist method.
In the end it will come to a federation made of instances with the same moderation policy. It will work because this policy will enforce the best content possible and will attract the people looking for quality content. So the people complaining about "defederation being a fascist practice" will stay on the old global federation, nothing will be forced upon them, while other people looking for quality will silently leave for instances with better federation standards. The most motivated people will bootstrap the concept, probably people passionate with hardware or another hobby.
Lemmy.world won't be on their federation list. Too bad for them but it's their fault for creating an account on a spammy instance.
This state where everyone federates with everyone (except criminals) comes to an end. We will have many different federations, not a single one.