  Impurity effects on the triangular antiferromagnets Ni$_{1-x}M_x$Ga$_2$S$_4$
($M=$ Mn, Fe, Co and Zn) are studied. The 2D frozen spin-disordered state of
NiGa$_2$S$_4$ is stable against the substitution of Zn$^{2+}$ ($S=0$) and
Heisenberg Fe$^{2+}$ ($S=2$) spins, and exhibits a $T^2$-dependent magnetic
specific heat, scaled by the Weiss temperature. In contrast, the substitutions
with Co$^{2+}$ ($S=3/2$) spin with Ising-like anisotropy and Heisenberg
Mn$^{2+}$ ($S=5/2$) spin induce a conventional spin glass phase. From these
comparisons, it is suggested that integer size of Heisenberg spins is important
to stabilize the 2D coherent behavior observed in the frozen spin-disordered
state.
