The piece in Girl's Guide to One Direction portrayed her as a voodoo doll, pointing out "crow's feet caused by old age" and advising her to "date boys your age, not your shoe size".
"I'm very aware about the voodoo doll article in a certain magazine and have been for some time. It was one of the most hurtful things I've ever read," Flack tweeted.
"But thankyou for all the nice supportive messages! I don't take notice of nasty things written to encourage bullying. I've never had so many nice messages in one afternoon!!! Me and my crows feet are thankful (sic)."
She added: "Oh and my neice Willow could write a more intelligent article and shes seven... rant over. I got some serious blonde hair to sort out."
Flack dated One Direction's Harry Styles earlier this year.