In order to disable nova crawlers on your maps you must follow these steps.
 
1.-Go to raw/maps/... and find the zombie_mapname.gsc file and copy it to your mods/mapname/maps/.. folder.
 
2.-Open the file and look for this function: zombie_school_ignore_spawner( spawner ) will have the name of your map at the beginning (mine is zombie_school) and replace it with the following:

zombie_school_ignore_spawner( spawner )
{
    // no power, no quads
    if ( !flag("power_on") )
    {
        if ( spawner.script_noteworthy == "quad_zombie_spawner" )
        {
            return true;
        }
    }
    if ( spawner.script_noteworthy == "quad_zombie_spawner" )
    {
        return true;
    }
    return false;
}

NOTE: Remember to change the name: "zombie_school" to the name of your map.