We exploit scanning-probe-controlled domain patterning in a ferroelectric top layer to induce nonvolatile modulation of the conduction characteristic of monolayer MoS_{2} between a transistor and a junction state. In the presence of a domain wall, MoS_{2} exhibits rectified I-V characteristics that are well described by the thermionic emission model. The induced Schottky barrier height Φ_{B}^{eff} varies from 0.38 to 0.57 eV and is tunable by a SiO_{2} global back gate, while the tuning range of Φ_{B}^{eff} depends sensitively on the conduction-band-tail trapping states. Our work points to a new route to achieving programmable functionalities in van der Waals materials and sheds light on the critical performance limiting factors in these hybrid systems.