Not with the way instance blocking works on Lemmy, unlike Mastodon which limits interactions from blocked domains lemmy doesn’t limit interactions at all through user blocks, it just makes them not appear.
Combined with the fact that instance blocking doesn’t even actually block the users from those instances. All it does is filter out every single community. If you read the page on join-lemmy.org it’ll tell you that clear as day.
Like I said for both of these reasons user-based instance blocks on Lemmy should not and cannot be considered an alternative to defederation.