You have a few options, you can delete the backup from within iTunes itself. This can be done by going to Edit -> Preferences -> Devices tab -> Backups. Highlighting the backup you wish to delete and hitting the Remove/Delete button.
Alternatively you can hard delete the files from their storage location. Depending on your OS is where they will be, namely here:
On a Mac:
On Windows XP:
C:\Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
On Windows Vista or 7:
Hope that helps!