I may be mistaken on the exact details of how it works on reddit, but allowing non-moderator users to prevent others from replying doesn't sound like a good idea. For comments, preventing a blocked user from replying directly in a child comment means they'll just reply in a sibling comment instead. They still get the last word, so the only thing accomplished is to mess up the threading a little bit.
For posts, preventing blocked users from replying gives the poster pseudo-moderator powers over replies. They can block anyone criticizing or disagreeing with them, giving them significant narrative control. Not exactly desirable.
Blocking should only be for filtering what the blocking user sees. It cannot be a substitute for proper moderation.