Fair. It’s not too hard, but most lemmy UIs make it a bit harder than it needs to be because they want to be a fancy JavaScript-ridden mess of html tags.
On old.lemmy.world it is supremely easy, you just use the element picker tool of uBlock to select all posts, add the ‘magic’ command :contains(reddit) to filter out the word you don’t want (in this case reddit), and you’ve got your filter. This would result in old.lemmy.world##.post:contains(reddit).
On lemmy.world it is trickier because it is the kind of HTML no sane person would write. Doing the above you end up with lemmy.world##div.mt-2.post-listing:contains(reddit) which is messy, and misses a line that is used to divide the posts. With some manual tuning you can first simplify the first part to ##.post-listing:contains(reddit) and then add :xpath(.|following::hr[1]) to get rid of the annoying line. This results in ##.post-listing:contains(reddit):xpath(.|following::hr[1]).