In a terminal as root, go to the directory /sys/class/leds. There will be several directories and you will have to find the right one. It will probably have phy in the name. In that directory there is a "file" called trigger. If you do cat trigger, you will get a list of all the options you can use with the currently selected one in [square brackets]. You just need to write the one you want back to the file. For example, echo phy0assoc > trigger should change the led to show whether or not the wifi is currently connected. Your mode name might be different. none will disable the led. You can either manually do this each boot or setup a script to do it automatically.