Block unwanted Gmail Texts
I faced a similar issue where some stranger sending me creepy texts using Gmail chat. I simply sent a SMS with the keyword "Stop", and after that I didn't get a single text from that email address on my phone number. I'm not entirely sure if it would work in 2019, but you can give it a try. Same thing is suggested in this old article: https://techwithlove.com/how-to-send-sms-from-gmail. So it might worth a try!

If supported by your carrier, then you can login to your account and block that email from sending you text messages. Someone suggested these steps on AndroidCentral forums to block text messages from the email and/or on Verizon:


Login to your account and hit MY VERIZON.
Choose VERIZON FAMILY SAFEGUARDS AND CONTROLS.
Choose INTERNET SPAM BLOCKING.
At the bottom of the number block list, choose BLOCK ALL TEXT
MESSAGES SENT FROM THE WEB and BLOCK ALL TEXT MESSAGES FROM EMAIL .
Save and close.


Source: https://forums.androidcentral.com/verizon-galaxy-s-iii/297351-how-block-texts-email-addresses-solved.html