Can't delete old Time Machine backups
On Lion, you can use tmutil to clean things up properly.

To delete backups one by one (snapshot by snapshot).

sudo tmutil delete /Volumes/drive_name/Backups.backupdb/mac_name/YYYY-MM-DD-hhmmss


If you are sure you have selected the correct path, you can delete all backups from that one Mac by not passing in the time and letting it delete the entire folder:

sudo tmutil delete /Volumes/drive_name/Backups.backupdb/mac_name


The sudo command needs your password (and it won't echo to the screen, so just type it and pause to be sure you're dating the correct files before pressing enter).