Are you referring to the thread preview that shows up when you move your cursor over the title of the thread? If so I do not believe there is a way to turn it off but if you don't move your cursor over the title it won't show up. Once you get use to it, it really is a neat feature. To be able to read the first few lines of a thread without having to open the thread is a real time saver. If you like what you read, you can open it and read the whole thing. If the first few lines don't interest you you can move on to another thread.