How to stop the LG phone launching preset applications when the phone is locked and inside your pocket?
NEW SOLUTION:

In the official LG update of Android (5.0.2) LG finally came themselves with an option to turn off the "feature".

Settings | System | Shortcut key





ORIGINAL so called a "solution":

After a long research, I am answering my question by stating that in Android 4 (the original version of the phone) it does not have a proper solution.

I called to LG helpdesk as well as wrote them emails. They always answered that "It's a feature, not a bug", the buttons events are hard-coded and that I can use a protective case if I want to avoid the accidental push.