is it an actual person, or is it an automated thing. If it is automated you can try replying "Stop" or "Quit". Someone may have signed you up online for something.
If its an actual person with a 10 digit phone number you can reply to them and tell them to knock it off. If its eating away at your minutes, call Rogers and tell them. If its not, you can make a custom ringtone to "none" and it wont ring when they text you.
It is annoying tough. I had this happen to me, but it turned out to be an automated thing, and replying "Stop" made the evil messages disappear.
It's probably not anything your carrier can stop, if you can't get the spam stopped, you might have to change your phone number. Not sure if there's anyother way!