First option, and the only one possible without a JavaScript hack,
would be to increase the iframe height to fit the height of your
content. That would work fine so long as the iframe content is
consistently the same height.
If you want to use JavaScript, you can either set up an event handler
that prevents the default action of the onscroll event (not certain
that this would even be possible), or maybe to load the external
content into a DIV using AJAX, but both of those options are beyond
the scope of this discussion list.
There doesn't exist any pure CSS way, as far as I know, to completely
prevent scrolling, even when using overflow:hidden.