Go to the Dialer (Phone) app. Type in 3845#*850# (AT&T version)
This will take you to the "Hidden Menu" Scroll all the way to the bottom. There is an option to turn Knock On/Off, off, there.
This answer found @ http://forums.androidcentral.com/lg-g3/413362-how-...
To get rid of the knock code, you will need to download an app called Activity launcher. Activity Launcher - Android Apps on Google Play
Once you have installed the app, at the top it´s going to say recent activity. Tap on that and change it to all activities. After it´s done loading, go to settings and then scroll all the way down until you see knock on. Tap on that and in the upper right-hand corner you will be able to disable it.
found @ https://www.quora.com/How-do-I-turn-off-the-Knock-...