First make sure that your browser has cookies enabled.
For Chrome: https://support.google.com/accounts/answer/61416?hl=en
For Firefox: https://support.mozilla.org/en-US/kb/enable-and-disable-cookies-website-preferences
For Internet Explorer: http://windows.microsoft.com/en-us/windows-vista/block-or-allow-cookies
To turn off the autoplay feature please click on the autoplay button. This located in the gear at the bottom of the player. If you do not see the gear. You should notice at the bottom right of the player there is copy that says "never autoplay" clicking on that should also disable the autoplay as long as your cookies are enabled.
Once you've clicked on the gear. Click on where it says autoplay on to turn it off. To make sure this worked please refresh the page. The video should no longer automatically play.