Antonyms for "rack up" include terms such as diminish, decrease, reduce, and diminish. To diminish means to make something smaller or weaker, while a decrease suggests a reduction in quantity, size, or intensity. To reduce indicates lowering something to a specific amount, and to shrink means to become smaller in size. The antonyms for "rack up" emphasize the opposite effects of accumulating, increasing, or adding to a particular quantity or amount. However, these contrasting terms can be equally valuable in different contexts and situations, where the focus is on either building up or reducing the quantity or intensity of something.