Cant remove stalled (inProgress) Time Machine backup

tmutil listbackups shows you a list of the backups found on your Time Machine disk.
To delete a backup shown in this list, run

tmutil delete /path/to/backup.inProgress

The only safe way to delete time machine files is to use tmutil (as explained in grg's answer). Using sudo rm -rf will seriously damage your backups. This is because time machine uses hard links to directories. Hard links to files are safe to delete. But a recursive delete of a hard link to a directory will actually delete the directory contents. So your partial backup may contain a hard link to an unchanged directory used in a previous backup. Deleting that hard link alone wouldn't cause damage, because your previous backup has its own hard link to the directory. But those two hard links point to the same directory, so when you do a recursive delete of the contents, you're actually deleting the contents of the directory that your previous backup was using. So now you've ruined your previous backup (and perhaps many previous backups). There's a reason why Backups.backupdb is locked down.