Do you mean 3G / mobile web?
Go to the phone dialer and type: ##7764726 then click call.
The phone will ask for your SPC which is usually six zeros.
Scroll down to the bottom where it says Data Call Settings and you can uncheck Data Call Enabled and you can also put a check in Disable At Startup if you wish.
To make this process easier download an app called AnyCut and use it to create a shortcut to Data Call Settings right on your home screen. Note that if you enable WiFI and then later disable WiFI data automatically gets enabled and the 3G icon will appear again so you have to remember to go back into Data Call Settings.
Hope this helps.