I'm not sure on the blocking options based on the information provided. You can forward it to 7726 (SPAM) and follow the instructions in the reply.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
Verizon offers a free limited number of slots to block numbers. They also offer a premium package with more slots. This can be accessed from within the My Verizon account webpage. If the message app bypassed this and allowed blocking, then the purpose of the free/premium offers would be null.