Update app.yaml

bigbluebutton-html5/private/config/public/app.yaml

@@ -18,7 +18,7 @@ app:
   copyright: "©2017 BigBlueButton Inc."
   html5ClientBuild: "HTML5_CLIENT_VERSION"
   defaultWelcomeMessage: Welcome to %%CONFNAME%%!<br /><br />For help on using BigBlueButton see these (short) <a href="event:http://www.bigbluebutton.org/content/videos"><u>tutorial videos</u></a>.<br /><br />To join the audio bridge click the gear icon (upper-right hand corner).  Use a headset to avoid causing background noise for others.<br /><br /><br />
-  lockOnJoin: false
+  lockOnJoin: true
   defaultWelcomeMessageFooter: This server is running a build of <a href="http://docs.bigbluebutton.org/1.1/overview.html" target="_blank"><u>BigBlueButton 1.1-beta</u></a>.
 
   # Name displayed in the brower URL