'Keeps down' means suppressing, controlling, or minimizing something. There are a plethora of synonyms that can be used in place of 'keeps down' depending on the context. Some of the common synonyms for 'keeps down' are repress, quell, subdue, restrain, curb, dampen, stifle, flatten, reduce, hold back, subjugate, smother, and silence. Each of these synonyms has a slightly different connotation, and employing the one that best suits the context is important. For instance, to suppress a riot, one could use force to restrain the agitators. Similarly, to keep down noise in a library, one could dampen the acoustics.