i also face such a problem..what i did was to take out the screw at the collar of joint, drill through the handle post..use a tool to set some screw threads..lastly replace the existing screw with a longer screw..it won't move anymore..

make sure you align the handle post before you drill..but if the handle post is slight off after the drilling, you can adjust the position by loosening the headset (i mean the piece of metal that you need a 10mm allen key to open)..