Can you tell me if there is a way of preventing access to Opera VPN using BT parental controls and/or BT smart hub.
Most unlikely. VPN for example Tor Browser and other tunnelling applications bypass these controls. Tor Browser goes right through any blocks set in my router and ISP filtering.
Using BT Parental controls, try blocking the following domain surfeasy.com
That is used by OperaVPN, which is not really a VPN, its just a proxy server.