The info I just posted in another thread linked below might also be of help to you. Simplified, once you have someone in your scenario, you have a checkbox option to allow newcomers. If you uncheck it, no one else can enter and I assume it removes it from the scenario list.
You can find this option under the LISTING OPTIONS button located on the top right of your screen when live in AM mode. You have the same options in masterless mode as in mastered mode. The button is just in a different location on the UI.
http://www.ryzom.com/forum/showthread.php?t=25775