1. Go to File->Script->Catalog.
2. Select the Registered tab. It will show list of scripts which has been registered.
3. Now select the script which you want to unregister and click on button.
Thanx. That worked.
I always build menus with commands. So I never really experimented with all the options in the Scripts Catalog. It takes too much screen real estate, as I am already short of space even on a 23" screen.