By default you are given the plan which charges you on the basis of your usage. They will charge you only if you use GPRS. You need not de-activate it. There are other plans for which you can use unlimited.
If you want to de-activate call the customer care.
Note - In the IOM (Internet On Mobile plan) you can only browse, your application on your mobile will not be able to connect to GPRS. If it does then you'll be charged.