Well that makes no sense, you're simply setting it to what it is, it's like if the code didn't do anything at all.
If you want to stop it altogether just try using
setVehicleTurretPosition ( veh, 0, 0 )
It might need some adjustments if you want it to face forwards.
Also, this:
getElementModel ( veh )
Will throw warnings if the player is not in a vehicle, just put simple
if not veh then return end
before it.