Yes, you can do this in the settings section of your device. It is important to note that if you completely disable the 3G or 4G ability of your device, then it may not work correctly. Unused data included in your plan will continue to roll over to the next month, for a maximum of 12 months.

• For Instructions on how to disable data on your iPhone check out this article on Apple's helpdesk here.
• For instructions on how to disable data on your Android device check out this article on Droid Life here.