A topological or geometric phase (GP) blockade can be introduced in molecular electron transfer processes if a conical intersection (CI) occurs between two charge configurations. The origin of the blockade is in the destructive interference of two pathways around the CI that acquire opposite GPs. Electron transfer quantum dynamics including and excluding GP have been modelled for the bis(methylene) adamantyl carbocation. These calculations have shown unambiguously that the CI topology and the induced GP are responsible for the transport blockade.