If schedule conference is always with MCU and MCU to dial-out participant (Endpoint), you may disable the call from Endpoint by using CPL or search rule configuration on VCS.
Basically only allow call from MCU to Endpoint but not from Endpoint to control ad-hoc call initiate from Endpoint.
This is not really clean solution as you may have more management operation, but can restrict call making from Endpoint quickly.
Of course you can configure exception Endpoint which can initiate the call as well.