A. The Torah only forbids “your” Chametz, but not “other people’s” chametz (Talmud Psachim 6b, based on the addition of “to you” in the verse). Chametz that you are responsible for, although it belongs to someone else, is also considered yours. If your roomate isn’t Jewish, or is Jewish and not (yet) concerned with Chametz – then you should just clean up your “half” and don’t worry about his/her stuff. As a courtesy you might ask them not to put freshly baked doughnuts or bagels in your face.