I cracked it, having deleted the applications folder in a fit of rage. Here's how to get rid of all the Nielsen shi*e:
My email to them:
Hello,
like countless other Mac owners, I am frustrated and offended by the presence of your software on my Mac. I didn't put it on my laptop, so your company has trespassed on my property. Your instructions for removal are useless. I have no folder called 'Nielsen' in my applications. It was the first thing I deleted. Still I get pop up messages asking me to trust the software, still I have your crap in my sound preferences.
I would be hugely grateful if you would send me the easiest and most effective way of getting rid of this annoying *****.
Their reply:
Thank you.
Thank you for contacting NetRatings Australia.
The Nielsen folder that was sitting in you Applications menu contained a file called 'Uninstall' that needs to be run to remove the software from your computer. Unfortunately if you have deleted the folder then you will need to reinstall the application in order to retrieve the files necessary for a full uninstallation.
As we do not have a record of you in our Australian database, we have set up a dummy account that you can use to log in and obtain another copy of the software:
Once you have logged in please select the option to 're-install'. Download and run the installer. Once the installation is complete the Nielsen folder should reappear in your Applications folder. Open the Nielsen folder and run the 'Uninstall' file. Shortly after the uninstall process has run you may be prompted to restart your computer.
I am now Nielsen-free.