Barsel - //Can a member of the Royal family stop this kind of reporting? Surely Prince William has got enough clout to do something about it. //
As advised, no-once can stop reporting simply because they may not like it - unless the papers break the law in terms of libel etc., they can print what they like.
As pixie said at 09:13, stories drive sales, and sales drive stories - if it became clear that the readership was tired of Princess Diana stories, they'd stop printing them, but there is clearly no sign of that happening any time soon.