I see many ways this could be done with various levels of efficiency, and this is true of any tag-based solution, be it web analytics, ad networks or even widgets such as Feedburner subscribers count and many others. Some people have already highlighted a couple of ways. Here's some more info:
- Disabling 3rd party cookie: obvious and easy, and that's why it's so important to use 1st party cookies, or at least what is called "friendly 3rd party cookies"!
- Antivirus/antispyware/ad blockers: also obvious and mostly the same thing as disabling 3rd party cookies because those tools will look at the 2o7.net used by Omniture (or whatever tool)... again, use 1st party cookies!
But then again, although 3rd party cookie blocking is easy, it's still not pandemic and there are "clean" ways to work around it (and I'm not talking about exploiting Flash Local Shared Objects to bypass cookie security!). I think cookie deletion is a much wider problem and it might have more impact on our analysis capabilities. Totally blocking a specific web analytics, yes, but would it be statistically significant? Surely not.