If you do not wish to receive chat messages from users you don't follow, you can control this feature from your settings by going to Profile > Settings > Privacy and turning on "Allow chat messages only from people you follow."
If your settings is set to off, users you don't follow can send a message request to you.
Message requests are never marked as read, and you can either accept or reject them.
Note: Messages that are inactive for over 179 days are automatically deleted.