Block ethernet port for single account

Hi, just wondered if it's possible to block access to the internet on another account on a computer, when an ethernet cable's plugged in.

I share a computer and we each pay for and use separate internet connections. I use the broadband, my brother uses a wifi dongle. However, on his account it automatically connects to the broadband when an ethernet cable is connected. If he disables it on his account, it disables access on mine too.

Is there a way of disabling access to the wired broadband on his account so that it will only connect to his wifi dongle, while still defaulting to the broadband through an ethernet cord on mine?

Sounds stupid, I know