Not really sure what you're talking about here. r/worldnews, r/news, r/UkraineWarVideoReport, r/CombatFootage, r/inthenews, r/RussiaUkraineWar2022, r/Ukraine, r/CrazyFuckingVideos, r/UkrainianConflict, and even r/Damthatsinteresting have posted about it today with thousands of upvotes each.
r/WorldNews even has a stickied megathread about today's news, and sorting by "Hot" on their sub only has a handful of posts not talking about the Wagner Group.
I do too, but this thread is about ways to filter or block content so I'm not really sure what that has to do with it?
I'd love a way to filter by keyword, which the Reddit Enhancement Suite and some of the 3rd party apps allowed. Maybe the upcoming Sync for Lemmy will port over its filters by domain, user, subreddit, flair, and keyword.
As for Reddit posts invading Lemmy, it seems like most of them are contained to c/reddit and c/RedditMigration, so blocking those two should fix most of OP's issue and that's easy enough to do without any extra tools.