Rogers store/Fido store reps are liars. I asked if I could disable data, they said "not really". After I bought my iPhone from a third-party store (Wireless Wave), I blocked data on my handset
and called Rogers and had them put a data block on my account.
So, yes: Rogers can block data on your account. Their in-store Reps will tell you they can't, because they make a bigger commission if they sell you a more expensive plan.
You can block data yourself, too, by installing a Fake APN... just visit
www.unlockit.co.nz on your iPhone browser and follow the prompts. Easy as pie. When you open Safari/Weather/etc. you get a message saying "could not connect to a wireless data service".