This post is not going to be much help, but I just wanted to share my experience.
I also tried to make my game isometric. I made 1/3 or more of the images needed and when I tested it I hated to. The character wobbled the screen around too much and over all I was unhappy with the end result.
Now I'm just making it a top down (straight up and down or left and right) game and am trying to add some extra elements to the gameplay to make it a bit more interesting.
As far as disableing N,S,E,W, I don't believe that it is possible. This engine most definetely lends itself to the top down (up,down,left,right) instead of the latter. It's too bad because i think that my game in particular looked better as isometric. Oh, well - back to work I go.
Also, just thought I'd add this in - if you leave all the directions then it will make it easier for the player to fight the baddies.
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(This message has been edited by Klatu (edited 08-17-2002).)