Here are the solutions to prevent them from playing :
Trick 1: Use Youtube Prevent Autoplay Greasemonkey Script
Just install the greasemonkey script and it will work like a charm . But you need to first download the greasemonkey addon , then only the script can be installed .
Download Greasemonkey Addon
Download Youtube Prevent Autoplay Script from userscripts.org
Trick 2: Use Stop Autoplay Firefox Addon
This addon will stop Youtube videos from auto-playing and also stop all embedded media from playing automatically. Once you click on the video, it will then load and then start playing.
Install Stop Autoplay Extension
Note : These will work only for Firefox users !
If , still there remains an alternative, feel free to suggest us .