I’m not the other person, but there’s no privacy setting for normal messages, in the iOS version at least. So it’s impossible to block messages from randoms.
The only way to do so is to have a username or phone number of recipient. Usernames are searchable if they are "Public Usernames". Phone number could be hidden from anyone.
I don't see how someone can contact somebody on the messenger yet without an ability to.. actually send a message. Sure, there could be an option to block everyone not in the contact list and use some other medium to relay the contact request[0] but most of the time this is useless for messenger.
But of course I have my POV skewed, because I prefer to use services defensively from the start so I had almost no unsolicited messages on Telegram, WhatsApp and Instagram.
[0] Hey, if you interested in $TOPIC contact me on Telegram, @username! But first send a couple of avian carriers with your phone number and username so I could add you to contacts first!