pop the aluminium part of the stem in a vice (cover the vice with some cloth first to prevent scratching it)
remover the stem bolt and wedge.
get 2 adjustable wrenches - put the first on the slanted end of the stem and the other on the top lock ring.
Then just tighten it up a little.
If that fails - do the opposite of the above (to take it apart) and put a little
lock-tight Blue semi-permanent other brands are available.
And tighten it all back up