It's a built in feature, so it's not easy.
The only way I can figure you can do that would be to limit the number of call appearances to just one. This would "break" the feature, not restrict it. There would still be a "Conf" button on all of the phones.
Without secondary call appearances, you can't get the "second dial tone" needed to create a conference. This would also break the Transfer button. It also needs a second call appearance to work.
Carpe dialem! (Seize the line!)