I unsafely removed a usb device that was attached to loop0 with losetup and could not delete nor detach loop0 afterwards.
/dev/loop0: :145606719 (/dev/sdb1)
losetup -d /dev/loop0
loop: can't delete device /dev/loop0: Device or resource busy
are you sure, that the device isn't busy? Have you ever tried fuser to determine an possible PID? Precautionary get all possible informations:
fuser -c /dev/loop0 fuser -d /dev/loop0 fuser -f /dev/loop0
try to stop the process, which could use /dev/loop0 if necessary use kill -9 or: try fuser -k to send the kill signal -> look at man fuser