Don't cover vents, period. It's just bad for obvious reasons, including those mentioned by other anons.

As for cooling, there's no good way to cool the room in question outside of getting a window air conditioning unit or fan, and you mentioned the roommate might be bothered by the noise (which sounds ridiculous, btw--if she wants the temperatures she likes whether warm or cold, she could learn to deal with noise.) If the heat issue is mainly a matter of sleeping, though, try sticking sheets or blankets in the fridge before bed.