In power networks, the flow is related to the generation/load injection by the bus (node) admittance matrix, which can be considered as a weighted graph Laplacian. This type of network flow model complicates the analysis of adding or deleting some connections for given node injection. Specifically, the present paper investigates the problem of mitigating the flow of geomagnetically induced currents (GICs) by blocking the transformer neutrals. Induced by solar storms, GICs cause half cycle saturation of transformers and consequently affect the power grid with harmonics and reactive power losses. The effects of blocking devices have been found to be mostly local at the substation level. To facilitate the analysis of blocking effects, a linearized approximation model is used by selecting the substations to block all high-voltage level transformers therein. For a given budget on the number of blocking devices, it can be cast as a binary constrained quadratic problem and further relaxed to a convex semidefinite program. The effectiveness of the proposed scheme has been demonstrated using both a 20-bus GIC test case.