There is this third party program called "CurrPorts"
it allows you to close a port on demand. I just tried it with 3 players (me and two external connections) and it works fine. it doesn't kick "both" people.
Download the program (the link for that is a bit low on the site I sent, under the "FeedBack" subtitle) <--- I don't think I am allowed to send direct download links.
extract it (it's a zip file)
scroll down to Terraria.exe
then find the IP that matches the person you want to kick (the Remote Port and Remote Address should match the server window) example if the IP in the server window is 184.108.40.206:55555 the remote port is 55555 and remote ip is 220.127.116.11
right click it and select "Close selected TCP connection"
and it's going to kick whoever you aimed for.
I have to explore it more to know more about it but it seems like a very fine program. Thanks to Nir Sofer Have a nice day! ^_^