A rate p:q block encoder is a dataword-to-codeword assignment from p-bit datawords to q-bit codewords, and the corresponding block decoder is the inverse of the encoder. When designing block encoders/decoders for constrained systems, often, more than 2^p codewords are available. In this paper, as o ur main contribution, we propose efficient heuristic computer algorithms to (i) eliminate the excess codewords; and (ii) to construct low hardwar e complexity block encoders/decoders. For (0, 4/4) and (0, 3/6) PRML constrain ts, block encoders/decoders generated using the proposed algorithms are com parable in complexity to human-generated encoders/decoders, but are signific antly simpler than lexicographical encoders/decoders (joint work with Brian Marcus).