If this is an iPhone telephone call, and iPhone Text message, or an Apple Messages message on an iPhone, iPad, or iPod Touch, then what you do is create a Contact for this person. The app where you receive the message, should have a way to create a contact from the information in the message.
For example, an iPhone call, you would look at "Recent", tap the ℹ (information button), and you will be given the option of creating a contact. Once you have a contact for this person, at the bottom of the contact page is a "Block this Caller".
If it is the iPhone, iPad, iPod Touch Messages app, then with the message displayed, there should be a "Details" button. That should then give you an ℹ (information button) which will allow you to create a contact, or add to an existing contact, and again at the bottom of the contact page is a "Block this Caller" button.
Actually on the ℹ (information page) at the bottom is also a Block this Caller button, but I like to actually collect these junk phone numbers, messages in a single contact I call "Junk". It has a very long list of blocked phone numbers that I can easily review all in one place.
If this is via Apple's Messages based messages, you can forward the message to
<imessage.spam@icloud.com>
<https://support.apple.com/kb/HT5821>
If this is a cell phone carrier text message, carriers in the United States allow you to report SPAM by forwarding the text to 7726.
In Messages, cell phone carrier Text messages appear in Green. Apple Messages protocol appear in Blue.
Now if this is Messages running on a Mac, I am not sure how you can block those.
