port blocking doesnt always block torrent activity. the best means of limiting torrent activity is to either limit the amount of bandwidth allocated for such activities, something you can do through your router with permission restrictions. you can also limit bandwidth at specific times. check your router manufacturer, the manual will have the info.
If you block the ports and disable NAP (MAP?) or UPnP then they should only be able to squeeze around 10-20k download speeds from their connections. While not disabling it completely, it would act as a bit of a speed bump.
LinkSYS uses UPnP, I forgot where it is located but it is definately something you can disable. Also, there is a Port Forwarding/Mapping section you can go to, to delete any info that may have been placed there not by yourself.
Also, I would change the router password. If you use LinkSys then 192.168.1.1 is the universal address to connect to the router settings which can be accessed by anyone connected to the network. Name: blank, pass: "admin" is then the default info to connect, and should be changed if you are worried about him getting into those settings. This isn't the wireless info to connect to the netwoprk, but the info to get "into" the router once connected to the network (where you previously changed that QoS info).
Check out this
site to see if your ISP is listed as one who blocks/limits torrents and use that as an excuse if your roommate asks why it's been acting slow. If his torrent app reports the ports as being blocked by the application (since you disabled it) let him know that the ISP has the ability to block ports on their end for your connection as well.
To take it even further, if he asks you to open a port, do so for a couple days and make sure he knows it is open (through checking his torrent app, don't let him look at the router), and then disable it again. Say that you opened it and the ISP must've blocked it again. From there you could take it to the next level. Then say you called the ISP and they suspected "business level activity" and suggested that you upgrad to the next tier of service, but that there is no way for you to pay a penny more for it. Have him pay the difference of the upgrade, and then you won't have to care what he does with the connection (if you don't truly care), and you won't feel bad about asking him nicely to limit download/upload speeds during the day, or when you using the net.
Even though I don't think you should feel bad asking him that now, "Hey man, I don't want to be a dick, but I was downloading some **** (for school/work) and it's going really slow. If you have any torrents going, could you please limit them to half speed, or pause them (for a few hours) or until tonight? That would be cool, thanks."