Just as with "over 18" banners, I expect that if all users self-certify as being non-californians the law won't apply to you. And that seems perfectly reasonable to me. Perhaps with the exception that I also think that a modal banner to the effect of "you acknowledge that you are now doing business in Utah" also ought to suffice.

If you actually sell things to Californians that's different. At that point, yeah, I think you _should_ be subject to California law. You're doing the equivalent of mail order business with a resident after all.