A plate can symbolize an ideal or goal. Any circle in a dream can indicate this, as circles have no beginning and no end, so are symbolic of the divine. For example, to dream of hitting someone on the head with a plate in a shopping mall, warns you of the need to be responsible (shopping mall) when it comes to pushing your ideals (plate) onto others.
A dirty plate indicates you need to cleanse your colon.
A plate of food shown in a negative way indicates food items you need to cut down or cut out of your diet.