Well, maybe I'm the only person left in the universe who doesn't have a txting plan, and thus I pay per text message. But, that's me, and I've gotten two spam txt messages in the past couple days, and I refuse to continue paying for them. Mind you, this started after I ordered something from Amazon.com - which has pissed me off incredibly to begin with and I would not be surprised if this txt spam comes somehow through them (it's all a conspiracy!).
Anyway, found a great article on the web on how to block cellphone spam, at least through AT&T and Verizon.
* AT&T: Log in at mymessages.wireless.att.com. Under Preferences, you’ll see the text-blocking and alias options. Here’s also where you can block messages from specific e-mail addresses or Web sites.
* Verizon Wireless: Log in at vtext.com. Under Text Messaging, click Preferences. Click Text Blocking. You’re offered choices to block text messages from e-mail or from the Web. Here again, you can block specific addresses or Web sites. (Here’s where you set up your aliases, too.)
Here's the full blog post which explains things much better than I ever could since I don't really understand it.