How can I turn off text messages from unknown callers?

I've been getting inundated with political texts on my iPhone. Many have been from the same area code. I've turned on Silence Calls from Unknown Callers, but I would like to know how the messages can be blocked. Silence Calls just doesn't announce I have a text message, but I still see the message.

The numbers are not in my contacts list, and I don't think I have to make them a contact in order to block them, right?

Thank you!