The big problem is that with text messages, the CONTENT is not scanned. So it cant tell what the actual text is.. cant tell your mothers text from one of these spam ones.
In how they are delivered? Possibly. IF it was set up as a mass text, with X many recpients at once.. Not sure if that is something visible or not on their end. I know that you as an end user can not see who else was on it. It it something they can see? If they can, maybe it can bet set up like some spam filter email settings, where it blocks mass mailings to multiple recepients. But thats all assuming that can be seen.. while the sending carrier may see the whole list.. it may be processed OUT from them pretty much as a one by one type thing.
Even comes down to a mass mailing secquential thing. text 555-1234
Another problem is the spammers might actually spoof cell numbers of existing rogers / fido customers so if rogers blocks the number that number might actually belong to you and might affect your ability to send sms to other people even though your not the spammer. You would not like that right? So im pretty sure they are utilizing other proven methods to block these messages but with any thing some are bound to slip thru the cracks. Or the spammers might be using legitimate prepaid cell numbers from another carrier or outside of canada and just recycle numbers often so they do not get caught.