Here to explain to you first why raw salt and rice can warding off evil forces if not, some people will say i'm telling a lie and bluf here...haha...

raw salt is made from sea water, when take from the sea and get it dry under the hot sun, it enough to absorb all the sunshine and its considered full of "yang " energy and enough to drive off " yin " energy meanwhile rice also the same process, its harvest under a hot sun and also its not a single rice but a lots amount of rice... So the " yang " energy quite strong also....

Those uninvited guest is form in " yin " energy so when their sense something which it strong with " yang " energy, its may leave.... Its all a natural theory in Generating and Overcoming 相生, 相剋 in Element theory...