Just Try:
@products_to_destroy = Product.where("marked = ? AND created_at < ?", false, 15.days.ago)
@products_to_destroy.destroy_all
OR
Product.destroy_all("marked = ? AND created_at < ?", false, 15.days.ago)
Ref: http://apidock.com/rails/ActiveRecord/Base/destroy_all/class
Hope it helps :)