As everyone has said, canceling AT&T accounts via the online method is the easiest way. I tried calling them and they didn't really couldn't do it unless I had the phone number associated with it. I've done this for my own AT&T new iPad and 2 other AT&T new iPads.
Question is, how do you access the VZW iPad account online? Aside from logging in through the iPad and calling VZW? Basically, similar to AT&T's method described above.