I unsafely removed a USB device that was attached to loop0 with losetup and could not delete nor detach loop0 afterwards.

losetup -a shows /dev/loop0: [0005]:145606719 (/dev/sdb1)

When I remount the device to /dev/sdb1, umount it and try losetup -d /dev/loop0, I will still get the same error loop: can't delete device /dev/loop0: Device or resource busy. Is there a way to get rid of the loop device?

You may have to also use dmsetup to remove the device mapping. Easiest way, if it doesn't interfere with any other mappings is to use a dmsetup remove_all.