How can I reformat a drive that contains an encrypted volume?
It turns out that Disk Utility didn’t want to touch a disk that contained an encrypted partition, but it had no issue with changing the encrypted partition itself. I clicked on the encrypted partition (not the drive) in the sidebar, and the “Erase” tab appeared on the right. From there I reformatted the disk to Mac OS Extended without encryption, and afterward Disk Utility had no hangups about reformatting or repartitioning the drive.