I bought a used phone and I am unable to use enterprise activation because I don't know the original owner's email and password. Is there a way to cancel the previous account or reset?

Just wipe the device.

If the device is locked, simply type the password in wrong 10 times. If it's not locked then, as suggested above, wipe the device by going to Options >Security Options >General Settings >Press the menu key and select "Wipe Handheld."