rich in STL,
Well, you can block a lot of the spam that is sent to your phone's email address, not cell to cell. I assume you are on the ATT network? If you do have a ATT SIM go to http://mymessages.wireless.att.com/ and register your phone number, then you can get into it and modify the message settings. This is an ATT message system but it seems to be disconnected from their main ATT system, like they forgot about it. Personally, the first time I saw this site I thought it was a fake site trying to get your info so I used a temp phone to test it out. If you look at the bottom of the site page it looks like they quit updating in 2007. If I go to the main site I can't create an account with my ATT number but on this system it let me signup and control the messages sent via this system. Give it a try, it works. I have my ATT EX124G registered and it blocks emails to my phone as I specify. It must control messages It cut done on my spam a lot.
Like I said, it doesn't block the spam texts from cell to cell, good luck with that! Now that I think about it, I haven't spam texts for a long time so it must be helping. Hope it helps! If you try it and it helps, post back.