If anyone wants to go back about 5 or 6 years, the internet was fully grown when this happened, and the rumors were flying even back then.
There's no question but that HHN did one "dry" night, then backtracked the next night but still didn't bring Jello shot girls back that year. At the time, JKR seemed a likely candidate--she'd been here a few times to inspect the original WWoHP and she had a bit of a rep for being, for lack of a better word, "prissy" (the no Coke backstage policy, making Universal replace the roofs ...). The story quickly spread.
That said, a couple years ago, I forget the source, but someone who I trusted to know said it wasn't JKR but rather some 3d party corporate VP. Which ruined a cool story, and could be wrong in and of itself, but means the original rumor may be off-base.