Reactive power injection of distributed energy resources (DERs) can be used to mitigate the voltage disruption caused by the power injection from renewable generation. However, how to control the amount of reactive power injection in the presence of both adversary and uncertain renewable-generation and load remains a challenging problem. In this paper, we formulate a robust optimization problem for solving the setpoint controlling the reactive power injection of DER devices, so that the maximum potential voltage disruption across a feeder caused by uncertain environment and adversary is minimized. Our formulation is general and can be applied to both an adaptive setpoint method (where the VAR injection is a function of the renewable generation) and a static setpoint method (where the VAR injection is a fixed). While this problem incurs exponential complexity, we develop a fast approximate solution. Our numerical results demonstrate that the adaptive method can maintain the voltage disruption to be within a small neighborhood of the nominal voltage over a larger set of uncertainty than the static setpoint method.