If you're not busy, why not go just to spend some time with your friend. They usually have food and games at those kinds of parties, so you might end up having fun. Your mere presence, even if you don't buy anything, makes your friend look better to her potential customers.
I would say to her, "Please don't send me your sales e-mails, since your products are not to my taste. However, if you text me and let me know what day your party is on, I might be able to make it."
If you don't want to attend the parties at all, I would just say to her, "I'm not interested, please take me off your mailing list."