We ended up with this same problem.... 42 GB of machine key files. So I wrote this powershell: RemoveMachineKeys.ps1. Took a while before it actually started deleting them, but once it did the script blazed through them pretty fast. I added protection from removing IIS machine keys.

I could not use the above answers that depended upon which user created the keys, as these keys were being created in a web site and had the same created by user. I also did not want to care about the application pool name if I did not have to.