That gets to be a bit of a finicky one. I've had trouble preventing that as well, and while I get it to work fine in most browsers, it doesn't work in every browser.
But the trick is to use CSS with overflow:auto on each post.
If you want, you can try to use the layout I've tweaked for my own guild's site:
http://www.poopinashoe.com/settings/pias/forumlayout/1/thread.htmlThe quote issue was a bug on my side. I tweaked something yesterday, and it broke the quoting. It's fixed now.