If you want to remove backups of your Windows Phone performed during ROM upgrades, close any instance of your Zune client and go to the C:\Users\<username>\AppData\Local\Microsoft\Windows Phone Update\<guid>.
Delete the directory called RestorePoint.
Restart your Zune client, no more information about ROM upgrades backups.
Screenshot before deletion
Screenshot after deletion
Due to a bug in the product, anonymous comments is currently disabled
Sorry about that