While a conference is in progress, you have the option to lock the 'room'. This will prevent additional participants from connecting via phone or web.
Via Web: While logged into your account, join the Online Meeting Room and click on the settings cog in the Menu and then click or tap on the 'Lock' button as shown below.
Via Phone: Ensure you have called in with the moderator PIN and press *5 from your phone's dial pad to prevent new participants from joining. To unlock the conference, just press *5 once more.