You can work around that problem in this way.
in your dialplan where people are calling into the conference add this (the name and val are arbitrary as long as its the same in scope of the current conference being called).
<action application="set" data="conference_name=myconf"/>
Then when you want to kick all, do this instead:
hupall conference_name=myconf
That will hangup all the people in the conference as well as the ones joining.
If you like having the kicked people hear the kicked message, you can use both commands in a row to kick the people regularly then wait a few sec to make sure they all heard the message and follow up with the hupall.
Another options would to use a global var or db app to set a field that locks the extension so you could lock it before you kick everyone so anyone who calls back would get a block message.