SMS messages are glorified e-mails.
You can send an e-mail to any e-mail address by sending a text message. You can send an e-mail to a phone number and it'll be received as a text message.
Therefore the stuff you can do with e-mail, such as spoof the sender, can be done with text messages.
To send a text message to a phone number, the e-mail address is the phone number and the domain is the the carriers SMS gateway. Wikipedia had a list of all the SMS gateways but they took it down. Someone mirrored it on their blog
here. He is also kind enough to give some instructions on how to use it.
To figure out what carrier someone has, use
http://whitepages.com/ for US numbers or
http://www.canada411.ca/ for Canadian numbers. There are probably others for whatever other country. Check the phonebook.
So let's say we found the number (012) 345-6789 has Sprint. Find Sprint In the list and you'll see it says
number@messaging.sprintpcs.com
so send an e-mail to
0123456789@messaging.sprintpcs.com and the person will receive it on their cellular telephone.
Of course, to SMS "Bomb" them, just send them lots of e-mails in a short period of time. This is easily done programmatically too. Just find an SMTP client in your desired language, every half decent language has at least one.