Plantronics + Polycom.
Now together as
It is necessary to disable the function of multiplies conferences, to prohibit the user from dialing a new number, if there is a call. How to do it? Version 6.1
Unless things have changes since I had this issue myself a while back, you need to remove the multipoint option key.
At least you can now diable the audio-add-on calls.
Another method would be to look at changing the dial plan and/or zones/sites setting to limit the number of calls allowed.