I wouldn't recommend disabling it or you might loose some functionality for advanced WiFi settings. Simply edit file /etc/xdg/autostart/nm-applet.desktop and add --no-indicator flag in Exec line:
Another way to solve this would be to comment out a line in the nm-applet.desktop file. In Terminal, do this:
sudo nano /etc/xdg/autostart/nm-applet.desktop
Then add a # to the line that looks like:
It should now look like:
You will probably need to log out, then log back in ...