Hi Alex,
If quenching in your case means you want to damp the system critically you could try to use the fix-viscous command. The critical damping constant can be approximated using the debye-frequency. The you will end up with a gamma of:
GAMMA = 1.0176713e-05 * T_DEBYE * MASS
You can then check your critical damping by monitoring the total energy: In the critical damping case you should see exactly two local maxima.
/Gerolf