For a graph $G$ in which vertices are either black or white, a zero forcing process is an iterative vertex color changing process such that the only white neighbor of a black vertex becomes black in the next time step. A zero forcing set is an initial subset of black vertices in a zero forcing process ultimately expands to include all vertices of the graph; otherwise we call its complement a zero blocking set. The zero blocking number $B(G)$ of $G$ is the minimum size of a zero blocking set. This paper determines zero blocking numbers of the union and the join of two graphs. It also determines all minimum zero blocking sets of hypercubes. Finally, a linear-time algorithm for the zero blocking numbers of trees is given.