The former Atomic Kitten singer wrote in her OK column last week that she thought Lennard needed "psychological help" after watching her on Celebrity Big Brother, prompting the 27-year-old to mock the "vile" star's past drug battles, weight struggles and her failed marriage to Brian McFadden on Twitter.
Responding in the new issue of the magazine, Katona wrote of Lennard: "To be honest, I don't even know who she is. She's just some bird who shagged Simon Cowell and went into the Big Brother house.
"At first I thought she was a crazy fan and blocked her on Twitter. I'm sorry, if other housemates have to hide knives from her there has to be something mentally unstable about her.
"A few centuries ago, she'd have been burned at the stake."
Lennard again took to Twitter after the publication of the column and described Katona as a "degenerate peasant" while mocking her past financial issues.
"Wow that woman's still talking about me?!" she asked. "She must really be struggling to make her rent again. #YAWN."
Explaining to a follower why she kept retaliating to Katona, Lennard continued: "I thought it was INCREDIBLY inappropriate for her to be commenting on ANYONE'S mental welfare.Particularly insensitively...she of ALL people should know better. End of.
"I attack defensively. That is the only time...jealousy is a disease kerry's just a little sick from it."
Lennard has also been condemned by Celebrity Big Brother rivals Danica Thrall and Rhian Sugden, who described her as "very unstable" and "psychotic" following their respective evictions.