  We use neutron scattering to study temperature dependent spin excitations in
insulating antiferromagnetic (AF) Rb$_{0.8}$Fe$_{1.6}$Se$_2$. In the
low-temperature AF state, spin waves can be accurately described by a local
moment Heisenberg Hamiltonian. On warming to around the Neel temperature of
T_N=500$ K, low-energy (E<30meV) paramagnetic spin excitations form
Lorentzian-like quasielastic peaks centered at the AF wave vectors associated
with spin waves, while high-energy (E>50meV) spin excitations become heavily
damped. Upon further warming to above the structural distortion temperature of
T_s=524K, the entire paramagnetic excitations become overdamped. These results
suggest that AF Rb$_{0.8}$Fe$_{1.6}$Se$_2$ is not a copper-oxide-like Mott
insulator, and has less electron correlations compared with metallic iron
pnictides and iron chalcogenides.
