How to preemptively block a non-contact on whatsapp I read this QA but it didn't specfically address my question. I also only found how to block a contact or someone that already sent you a message.

Say I have blocked numbers on my phone. They are not contacts. I don't want to hear from them. I don't want them to message me - ever.

How can I block someone when there's no chat between myself and them? (avoid potential harassment)

Or, alternatively, can I make it to only receive messages from people on my list?
No.

On Whatsapp you can't block people just by their usernames. You must open the chat between you two and block the person from the right-top menu. However, you can add that number as a contact, open the Whatsapp chat screen (you don't have to chat them. Just open their chat screen.) block them and then delete them from your contacts again. 

And no again.

Whatsapp doesn't support a feature like "Receive texts from my contacts only." But you can reduce what people who aren't in your contacts can see about your profile (Last seen, Profile photo, Status)