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.
Make sure to check back after a few days to see if Reddit illegally restores your posts, and file a formal deletion request in writing with support if they do so you can forward their non-compliance to the California AG per the California Consumer Privacy Act (CCPA) which they are required to follow by being headquartered in Silicon Valley. They've been restoring posts left and right already, falsely thinking they "own" us, that we won't notice they're breaking the law, and that we won't hold them accountable in large enough numbers to matter.