I realise this is an old question, but I'll answer incase anyone else has the same question.
Summary: It *is* possible to delete individual old backups, but only by using Terminal.
Here is what you need to do (on the machine that is being backed up, not a different machine):
1. Open Terminal
2. Become root (in order to have permission to do what you’re about to do), by typing:
sudo su -
... and then typing your password when prompted.
2. Obtain a list of backups by typing:
tmutil listbackups
This may take a few minutes. You will get a list like this:
/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2015-01-02-200304
/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2015-01-09-001847
/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2015-01-16-005850
/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2015-01-23-003211
…
/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2017-10-19-171147
/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2017-10-20-121353
/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2017-10-20-144607
/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2017-10-20-164616
3. (optional) If you want to check how much space will be freed up by deleting a backup, type:
tmutil uniquesize "/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2015-01-02-200304"
You’ll get a response like this, after a while:
908.9M
/Volumes/Time Machine Backups/Backups.backupdb/Xxx’s iMac/2015-01-02-200304
4. Delete an individual backup by typing:
tmutil delete "/Volumes/Time Machine Backups/Backups.backupdb/Xxx's iMac/2015-01-02-200304"
Again, this will take a while. Do this for each backup that you want to delete, or use wildcards if you’re confident that you know how to.
5. Unmount/eject the backup set by typing:
hdiutil detach /Volumes/Time\ Machine\ Backups
6. Browse to the location that Time Machine stores your backups. It should be a folder called Backups, containing an item with the same name as your machine. Select it and press CMD-C to copy it’s path.
7. Switch back to finder and type the following, pasting the path at the end using CMD-V:
hdiutil compact
e.g.
hdiutil compact /Volumes/Backups/Xxx\’s\ iMac.sparsebundle
This will usually take a long time, but when it completes you will have freed up some space on the disk that you use for your backups.