I would absolutely cut them back and save the seeds to sow or wintersow. Mine get rust each year, but it is not noticeable unless you get up close. It could be that your particular cultivar is not rust-resistant, or it could be that the weather conditions were such that rust was more prevalent this year. It may be that if you can increase air circulation to the back of the border, that may help.
If the cultivar is not resistant then the new plants, sown with those seeds, will likely retain that undesirable genetic trait, but it won't be the seeds that are "infected" so to speak. At least that is my understanding. I am in a much warmer clime than you, so take my comments with that in mind. You are "up there" with the renegade gardener, I do love his stuff.
Laurie p.s. I only have a few hollyhocks, but I am encouraged by your report that you had a bumper crop after 5 yrs. I keep throwing seeds at the back of the border.