Cats exhibit this kind of behaviour for all kinds of reasons, not always logical ones at that! I have had a cat behave in a similar way simply because she didn't like the person in question. Whatever the reson the easiest way to train the cat not to do it again (and they will, again and again, it's very difficult to remove all scent from a fouled area and they will use the same spot in the future of given the opportunity) is to either retrain them to use a litter tray (if you allow them to) OR to exclude the cat from the area she has taken a liking to. To retrain simply put the litter tray in the spot the cat has chosen as its new toilet and they SHOULD happily use the tray, albeit somewhere you don't want it. After a couple of days the tray use should have imprinted and you can move the tray to where you DO want it and the cat should follow. To exclude the cat from the area simply tell your room mate t keep their door shut!