Application of conducting ferroelectric domain walls (DW) as functional elements may facilitate development of conceptually new resistive switching devices. In a conventional approach, several orders of magnitude change in resistance can be achieved by controlling the DWs density using super-coercive voltage. However, a deleterious characteristic of this approach is high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach based on tuning the conductivity of DWs themselves rather than on domain rearrangement. Using LiNbO3 capacitors with graphene, we show that resistance of a device set to a polydomain state can be continuously tuned by application of sub-coercive voltage. The tuning mechanism is based on the reversible transition between the conducting and insulating states of DWs. The developed approach allows an energy-efficient control of resistance without the need for domain structure modification. The developed memristive devices are promising for multi-level memories and neuromorphic computing applications.