Do you refer to the spike in the switch ON? This is due to the parasitic capacity of the inductance.
You can supress it with a LP filter after the sense resistor.
8)If you talking about the current spike which has to be filtered at the current limit pin, then you have to use an RC filter.
If you are talking about the Ripple spike at the output a small bead with a small capacitor at the output will kill the noise .
If you are talking about killing the spike at the drain of the main switching device, use RC snubber across the main Switching element and also across the output rectifier