I rebooted again and this time was able to reformat. If at first you don't succeed try, try again.

So...
I had to reboot once before I could disable swap, once after disabling it to verify that it was disabled and twice more before I could format the memory card. I think the last reboot was a soft reboot, but I don't remember, because I was just messing around. I didn't expect it to work, and I was going to do it from my computer when I got home from work. I reformatted using the tool in the file manager.