Find the setting and configuration options in the online, video Web site that hosts the video you are playing. This option is usually accessed by right-clicking anywhere within the video's playback button, or the video itself. Some may also have extra buttons found within the browser window, or at the menu buttons located on the upper-left area of the screen.
Click the "Stop Download" button from the Web site's "Setting and Configuration" options. Some Web sites, like You Tube, automatically starts playback and buffering once the browser window, containing the video, opens. In the case of You Tube, you can easily find the "Stop Download" button by right-clicking anywhere within the video's playback button, or the video itself. There is a difference between simply pressing the pause button in your playback button, and pressing the "Stop Download" button. The former only stops the video playback, but the buffering continues, while the "Stop Download" button stops not only the video playback, but also the buffering process.
Close the Web page if the Web site doesn't have a "Stop Download" button available. In such case, this is your only choice to readily stop the buffering process. Since many Web sites don't automatically play the video, and they require you to first click the "Play" button before the buffering starts, just don't press the "Play" button yet. This is an option if you don't want to start with the buffering, but you want to keep the Web page with the video still open in your browser.
Check if the video successfully stops buffering. If the progress bar shows that the buffering continues, you may simply close the browser window, and just open it at a later time when you are ready to play the video. Another option is to refresh the browser window. This way, you can keep the browser window open. However, this only works if the site you are using doesn't play videos automatically upon loading of the Web page, which is the case when using You Tube.