Using Axel Foley's method - but adding another step - below I managed to block the text.
Fist, go into messages and open the thread from the annoying text
Click on the 'face' icon at the very top to show the info option.
Click on info
Now you will see the phone number very big (this is the screen where you'd normally see block but it's not there)
And another 'info' button click on that.
Then add the phone number to your contacts and label it 'block me' or something easy to find.
Now use Axel's first method Settings ➔ Phone ➔ Blocked Contacts ➔ Add New. - you can search for the contact and block it.