A number of cell phone devices have a shortcut key on the dial pad/keyboard for getting instant access to the Web. If you have activated GPRS on your phone and you accidentally press this key, you will be charged as per your data plan. If you don't need GPRS service, you can simply deactivate it and remove it from your account so you're not charged for a service you don't even require.
Log in to your carrier's website using your account username and password. Navigate to the "Services" page that contains all the services and features you have added to your account. Scroll down the list, and find "GPRS"/"EDGE" or "Mobile Internet" (or a similar header). After clicking on it, click "Disable" or "Remove."
Contact your network carrier's customer service department via phone if you don't find this option on the website. Tell the representative that you want to disable the GPRS feature that has been added to your account. The representative will then remove the feature and you will not be able to access it on the web browser.
Enter an incorrect GPRS setting on the handset. Go to "Menu" > "Connectivity/Connections" > "Internet" > "Data Accounts/Internet Accounts" (look for similar options on different phone models). When you choose the Internet account that your carrier is using at the moment, for instance AT&T MEdia Net, change its username or APN name to anything else. By changing any of the settings from their default values, the phone will not be able to access the Internet and you won't be charged for accidental use.