I have the same problem, LG and their stupid toggle buttons. There's no other way for me to turn down the display brightness down and it gives me a headache, so I literally wear my sunglasses at night, that and a handy program called F.lux helps somewhat :D
For me the toggle button lasted about two years just as my warranty ended the thing broke and I can't get no options and my volume is stuck pretty high but I only use for the PC anyway, which has volume control. Maybe there's some way to override the settings some how!
I wouldn't mind cracking this monitor open but I have nothing to replace the toggle button with, not sure if I can get a replacement or take out an old or similar model, not even sure how to take the thing apart. If anybody has the slightest idea as to go about it let me know?
Otherwise it's been a great monitor but I'm at the point now where its gonna get fixed one way or another even if It means breaking the thing. Muhahaha payback sound sweet....