  The proton decays too rapidly in supersymmetric theories if a dimension-4
operator \bar{5}.10.\bar{5} exists in the superpotential. The conventional idea
is to impose the R-parity to kill this operator with a stable lightest
supersymmetry particle (LSP) as a direct consequence. However, the SUSY-zero
mechanism is also able to kill the operator without an unbroken R-parity. In
this article, we provide a firm theoretical justification for the absence of
the dimension-4 proton decay operator under the SUSY-zero mechanism, by using
some input from string theory. The LSP may be unstable without the R-parity
and, indeed, some dimension-5 R-parity violating operators may be generated in
effective theories. This suggests that the dark matter is an axion in this
string theory inspired model. An insight on the SUSY-zero mechanism is also
obtained.
