Spam Folder, how to use

When an email is added to the spam folder, is that sender blocked? Or is that sender’s mail automatically put in the Spam folder?

If neither of these happy, how do I block senders?

what happens depends on what, if anything, that your provider does with items you put in your spam folder. k9 has no role here. you can check your provider’s documentation (or ask them) for what they say they do.