Do you have an iCloud account setup on your iPad? If so go to your settings and scroll down to iCloud. Now in there turn off the "Contacts" so that your contacts do not sync with iCloud anymore. Once you do that you will get a popup asking if you want to keep the contacts on your iPad or delete them.
If you do not have iCloud setup on your iPad then you must have your contacts setup to sync with your iTunes account. Hook up your iPad to your PC or Mac that has your iTunes account via the USB cable. Open up iTunes and select your iPad under the 'Devices" on the left hand column. Go to the "Info" blade (towards the top center of the screen) and then un-check the setting for "Sync Contacts With". Now press the "Apply" button on the bottom right hand side of the screen to have the settings applied. This should remove all the contacts from your iPad.
One good tool recomended to erase iPad data, a professional iOS Data Eraser software. This kind of software can easily find the deleted files on your iPhone/iPad/iPod and then remove them forever from your iPhone/iPad/iPod. You can learn how to delete contacts from iPhone easily here.