I know what you mean Echo. I had the same thing happen to me last time I played ZM. Some random guy came in, and acted like he owned the place while I was in the middle of setting up the barricade. I was so tempted to type rank.. Instead I let him build the cade, and watched him get tagged.. Got him and the invading zombie stuck and killed them both, and built the cade myself. Was pretty funny actually.
As for a private ZM server?.. Naw. I heard that was what gs-zm(or whatever) was basically. And they obviously aren't around anymore.
What seems to make the current ZM such a success is the mix of newcomers, regulars, and pro admins. It keeps the servers all alive.
Though most of those admins have gone off to college.. X_X I need admin again.