But the question is do you want to, or should you?
M$ regards this as a legitimate tool and is performing its required function.
Many do not agree, if you use a third party firewall, these can be set to block such actions, however Windows Firewall won't (obviously)
EDIT: At least I suspect it won't, I have no way of testing this myself, nor any inclination to do so. I would be most surprised to find out that you can configure Windows Firewall to block M$s own products, but there is no technical reason why not, other firewalls have no qualms about that.
A Google search for WGA Notify removal
like this:
http://tinyurl.com/yhkl8d
will get you loads of hits on the subject.
here is one tool, from SoftPedias site:
http://tinyurl.com/gwhhv
Although I do not approve of M$s malware on my machines, and I refuse to let it in at all, that is my [i]personal security choice[/i], I must point out that it is an officially approved tool from M$, used in it's attempt to combat piracy.
[b]Be warned that if you have automatic updates on, it will get re-installed next time that happens.[/b]
I have my Automatic Update settings set to "Notify me but don't automatically download or install them"
I then scrutinise each download and reject those I don't want.