In some cases, deleting backups from the Finder won't work; you also may not be able to use the rm command to delete these backups, because they are handled in a special way. Finally, even if all goes well, you may want to delete backups on a remote Mac's Time Machine disk.
There is an executable accessible from the command line that lets you delete these backups. To use this with Mountain Lion, run the following command:
sudo /System/Library/Extensions/TMSafetyNet.kext/Helpers/bypass rm -rfv /Volumes/[disk]/Backups.backupdb/[path]
This command works with Lion:
sudo /System/Library/Extensions/TMSafetyNet.kext/Contents/MacOS/bypass rm -rfv /Volumes/[disk]/Backups.backupdb/[path]
Terminal will display a list of the files as they are deleted.
Found on Stack Exchange. Note that I haven't tested this with Lion.

