i spend most of day in school, office, and my room where wi-fi is available, so i dont want to pay extra 30 bucks for data plan which i dont really need.
actually yes, you can. I told them that I switched my phone and they just took it off. Mind you, I did eventually sell my iPhone on ebay a few days later (USED FOR 465 !! WOOT) only for profit, but I would just be careful, you dont want to rack up internet charges. they didnt even charge me for a half months internet use. my bill I got on the 19th was 58.85 and I have 450 mins (39.99) plus ulim messaging (19.99).
If you do cancel your data plan, make sure to have the carrier completely block data connection. Otherwise you could rack up a huge bill if you think you're using wifi, but the wifi connection got dropped or broken.