Don't the Health and Safety regulations say that in case of a fire you have to know who is in and who is out. Otherwise, (God forbid) is there is a fire, the poor firemen will be looking for a person who is not there. Maybe this is the way you need to slant it. I stayed in many a hotel where you have to hand in your key if you go out for the day. If it is a nice family run B&B, it is only courteous to say, from the client, that you don't want breakfast. The card, with Rules of the House, seems like the best idea.