as in indian sandstone? if so you could take the high spots off with a grinder but would look naff, if its a big lump i usually knock it off with a punch chisel or a pitcher, not often i do that usualy cast the really rough ones aside and try and use them for cuts.
Ok well the chisel is the only answer. Don't forget though there is a top and bottom to Indian sandstone and the bottom is usually lumpier check the tapers on the side of the slab to ensure you are laying the stone correctly.