How to disable automatic brightness setting on LG monitors?

The option is not in the main menu, but has a dedicated button here:

Click the button enough times to select OFF.

I have the LG 24m45 and I was having the same issue. I looked all over for the menu to change it, however it is actually in a special menu-which is an awful idea. I found the separate menu by pressing down the function(FUNC./) key twice and the menu popped up. Hop this fixes your problem!

In my LG monitor, which I purchased a few weeks before, this feature (automatically changing the brightness once every while) was annoying.

I found that I can disable this feature by setting the following option:

Menu -> Settings -> General -> Smart Energy saving = OFF