We have had this happen to us a couple of times were only 90 feet not the mega yacht in pic .People just are not thinking when they drop anchor maybe because big boats are not so common. Usually after an hour they sort of figure out the swing of our boat and move (98 % of them) .
But there is always one that doesn't clue in
We hale them on the radio
asking them to move to a safe distance usually Coast Guard is listening, if the conversation goes on the Guard steps in.
If that doesn't do it you turn up the music
and fire up your gennys just to let them know it will not be a quiet night.
if all else fails break out the shotguns and the skeet shooter .