You know, they old forum would do that on long posts and I never liked it because I feard that many people would not see the scroll bar and would miss like my last two whole paragraphs. There is not option to disable that, but I can view the page source and match the code with one of the source scripts and uncode it from the forum.
check in your style sheet (style.css)
for the .post classmaybe it's put like this:
#div .post{...}
or
div.post{...}
or .post{...}
