If you're syncing calendars with an iPad/iPhone/iPod via iTunes, you can overwrite the device, as follows:
- Launch iTunes, dock your device, and select it in the iTunes source list.
- Click the Info tab.
- Select the "Sync iCal calendars" checkbox.
- Scroll to the Advanced section on the bottom and select the "Calendars" checkbox under "Replace information on this device".
- Click Apply.
