Welcome to this user forum for BT Retail phone and broadband customers.
You cannot do anything on the BT Home hubs.
The only option is the BT Parental controls, but they only work if devices are using BTs DNS servers.
My kids are using VPN to dodge the parental filters... How to block VPNs on BT Hub2
You take away the devices that they are using until they learn the house rules.
Another option (on a Windows PC) is to set up another account with administrator privileges, change their account to standard user and remove the VPN software. If they try to re-install it they'll require the admin password.
I've always set up my kids' PC's like this so they can't install anything I don't want them to have on there.
Trying to use technology to alter behaviour is not going to work. You’ll be frustrated and your children will win. Engage with them. https://www.internetmatters.org/issues/inappropriate-content/protect-your-child/