Jst threaten him damn it, theres only one of him. Get all lary with him or jst disconnect him from the router/internet when starts downloading during the day. Problems like this are best solved with a bit of aggression.
I know cause im in exactly in the same situation, the only problem i have is they are all spacs and dont know when they are downloading so it makes it hard to tell whos downloading what.
Trust me
Alternatively u cud try being really nice and pussy foot around the guy thats downloading and let him make you all look like mugs cause hes getting exactly what he wants.