What should I do about this?

I had planned to take the December SAT as a last resort, in case I didn’t get my target score on the September ACT (my third try). I got the score I wanted on the September ACT, so I decided I didn’t need to write the SAT. However, there seems to be no way to cancel my registration. What should I do? Do I just not show up on test day?

Yup . . . you just don’t show up. My son did that as well. You don’t get your registration fee back but otherwise it’s no big deal.