Okay, I'll assume you only answered the first if the two question, based on the second being more improbable of being correct.
But what you need to do is;
when any select box is changed, you trigger a function that disables the others. I might write some example code for that later on.
However:
A much simpler way and the one I would choose to use as it avoids the unnecessary reliance of javascript, is to use a single select element with <
optgroup> elements separating the mutually exclusive choices. This may of course require some layout changes but given that this IS the "Web Design" section of the forum it seems to be an eminently more suitable solution.