Reply to the message "STOP" in all capital letters. You should get a message back from them saying "All services from *** have been stopped." or something like that. If you don't get that response, don't worry about it, it should still have stopped.
In the case that you keep receiving them, repeat what I just said earlier, except instead of "STOP" try "END".
If niether of these work, contact the Rogers Canadian Wireless Inquiries phone line at 1-888-764-3771 and ask for assistance. It should be free to call this number as long as you are still in Canada when you call. Have your phone number and payer-information ready (full name and date of birth). Also, make sure you call from a phone line other than the one you are inquiring about, otherwise they can't access your phone (if you're on it).
I hope this work out for you, have a great day!
-Gordon
