Go to (within LXQt menu): Preferences~LXQt Settings~Session Settings

Two methods depending on your preference.

First Method: Under basic settings: uncheck the box for desktop and click stop.  This will place openbox in charge of your desktop (or whatever window manager you are using XFWM, KWin etc...), if it is openbox you will then need to configure it to handle your wallpaper: http://openbox.org/wiki/Help:Configuration

Second Method:  Create an empty folder on the desktop (give it a name, ex: blank desktop), then from Session Settings click User Directories and look for the section at the top "Desktop", click the folder icon and have it point to your new /home/user/Desktop/[blank desktop] folder.  After this you should now be the proud owner of a clean desktop with no folders or icons, and PCManFM still in control of the desktop (wallpapers etc...). 

To add icons back to your desktop (why would you though):
If first method then: check Desktop and click start

If second method then: click the folder icon and have it point back to /home/user/Desktop

Enjoy the journey, happy computing.