Miss Manners: We requested no presents, but she said she was going to bring a gift card regardless

DEAR MISS MANNERS: My father, my three siblings and I are having a surprise 80th birthday party for my mother. One of her cousins responded that they were coming, but also asked what restaurant my mom likes, because she wants to bring a gift card for her.
My parents don’t need anything and I know my mom would prefer not to receive gifts. I politely responded that it was a very nice thought, but no gifts were necessary. She said that I should tell her the name of a restaurant, because she was going to bring a gift card regardless.
Is it rude on my part to ask people not to bring gifts? Or should she back down?
GENTLE READER: No. Yes. But isn’t your real question: How do I get this cousin to stop pestering me?
Miss Manners has an answer for that one, too, which is, “You know, I’m not sure she would actually use it.”
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(Please send your questions to Miss Manners at her website, www.missmanners.com; to her email, [email protected]; or through postal mail to Miss Manners, Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO 64106.)