I suggest the
switchlight command...
Get the object ID of the Streetlamp (click the
Show ID's button and zoom in far enough to pick out the streetlamp) and use a line like this:
(Object 2995) switchlight "OFF"
Where 2995 is the number of your streetlight.
Just note you need Resistance to do this... If you only have v1.46 there is another way, but it's a tad more complicated