IF that is the case.. likely then something either account wise.. or again, someone MAC cloning the modem.
IF you are going to do the turn off thing.. you need to inform rogers first. Call in, escalate it, and say to please leave a note on the account, that i will be having it disconected for the period of X to Y 24 hour range.
Then, if there is the usage durring that time.. they have prior knoledge that it was disconected. (can even unplug it while on the phone with the person, and make sure they are NOT able to see the unit.... again, if you do, and they CAN.. well there is another problem like the clonging going on )
They usually wont take this AFTER the fact.. just as its to easy for someone to say go on a download binge, then after the fact call in and say "Oh mine was unpluged, it wasnt me"