If you are currently syncing with a service like MobileMe or other contacts, go to you server account in Settings >> Mail, Contacts, Calendars >> Accounts and turn off the Contacts sync. It should give you an option to delete all your contacts form the iPad.
Similarly, if you create a new account that syncs contacts you should be given an option to merge the existing ones, or overwrite new ones. You can use this if you have an account that has a contact address book that is empty. For instance, I have a Yahoo email account. I could sync contacts from it, but that address book is empty, so I could replace all my existing contacts with the empty one. This will only work if there is no account already being synced with.
Another option would be to use iTunes. Connect your device to the computer, go to iTunes, and select your iPad from the Devices list. Go to the Info tab at the top. Under Sync Address Book Contacts, choose any available contact sync source. Now chose Selected Groups, and make sure all the group checkboxes are NOT checked.
Go to the bottom of the page and in the Advanced box where it says Replace information on this iPad, check the Contacts box.
Sync.
Be sure to go back and turn off the Contact syncing when you are done.