If it is a legit company, texting STOP may help, but if they are a spammer type, that will just let them know you are reading them (like responding to spam email), and you'll get more.
Google the number to see what comes up.
Try calling your carrier. Some only offer all-or-nothing blocking though.
I have a cheap dumb phone (the cheap T-Mobile Samsung flip - T169? not sure of the model), it does allow me to block a call (and I assume texts from that call, but I'm not sure). So another option is to get a new phone that will block specific text numbers. Post to howardforums and you'll probably get a fast reply, if none from here.