Until now, a simple trick to stop buffering a YouTube video was to fast forward to the end of the video. The good news is that YouTube added an option to stop the download: right-click on the video and click on "Stop download".
YouTube's help center has more information about buffering. "The YouTube video player downloads a video as it plays. A buffer is a section of memory in your computer which allows for the simultaneous writing and reading of information -- on YouTube the buffered section is represented by the red section of the video timeline. The YouTube video player reads video information from one section of the buffer while writing to another. This kind of multitasking allows for smoother playback of video during a continuous process of downloading which is especially helpful for slower connections. "