Received a spam text in Messages on my Mac and had to search for how to block it. Found Block messages in Messages on Mac - Apple Support: select the conversation, choose Delete and then you can Delete and Report Spam.
@alanralph I use Messages quite a lot and will often get Messages from unknown senders that I do need to see, such as verification numbers, appointment reminders and the like. What really disappointed me was that as a veteran Mac user I had to search for the solution I needed.