is it possible for me to cancel the data plan once I buy the iphone??
Depends on the carrier.
If you're on Rogers like the OP, then:
1) A data plan is "highly recommended" at the time of purchase, but it is not a mandatory condition of the purchase. Choosing not to purchase a data plan may effect the up-front price you pay for the iPhone.
2) If at time of purchase you opt for one of Rogers' "normal" data plans on a fixed term as part of your subsidy eligibility, then an early termination fee of $5 per remaining month in the data term you agreed to ($100 maximum, $25 minimum) applies
even if you maintain the voice portion of the contract.
3) If you opt for one of the "special" iPhone rate plans where voice and data are both included in a single subscription, then switching to a different price plan which is voice-only, does not incur an early termination fee; however, switching from any price plan above $30 per month to a price plan below $30 per month incurs a downgrade fee of $50.
If you're with AT&T, then their terms of service state that if at any time they detect you're using an iPhone on their network without an eligible data plan, then an eligible data plan will be added to your account automatically.