Yes, you can block text messages. On the Sprint/Nextel network, you do it on sprint.com - sign in, the on the bottom right of screen, click on Block Text Messages. There are 3 options to block. I use a blocked list where I input the numbers from senders I do not want to hear from - currently only one is there from a spammer.
From other networks, I'm sure you can go to their site and do it from there.