Umm, the roommate is away, the router etc are in his room. It is a locked door.
You flip the power. It is pitch black. You are likely to be eaten by a grue.
Seriously, this is not
a. A social engineering problem
b. A network engineering problem
It is a simple, "I need to get into his room and unplug his laptop" engineering problem. If you want to take the brute force approach simply either pop the hinges on his door (assuming that they are on the outside) or just kick it in/pry it open.
I know that sounds heavy handed but there are times when it boils down to the simplest choice. If you have access to the router admin then yes, disable his machine in there.
If the poster had the knowledge to be able to poison ARP, then they probably wouldn't have been asking this question. (No offense intended)
There were some useful suggestion re: flipping the power/breaker for the house/his room and letting the battery run down, but as for re-routing the coax cable to somewhere else and bringing up another cable modem/router. Come on people this problem isn't rocket science.
My opinion is that if your roommate is inconsiderate enough that he leaves his torrent client running unrestricted and unthrottled while he isn't there, then someone has to have words with him about fair usage of a shared resource or split the bill differently (this is a bad road to go down unless everyone agrees to it, I think that heavy users of the network should pay more and people that don't use it should pay a nominal amount (couple of dollars a month, literally $2$3 this means that if they do start to use it there has to be another agreement between everyone as to proportional usage).
Of course, all of my points are based on the assumption that it is a shared internet bill. If he's the guy paying for the bandwidth then he can use it whatever way he wants. If this is the case then suck it up and be glad he doesn't just switch it off when he's not there.