Don't you consider they'll be smart enough to ask someone else to take a look at it from an IP address range you're not currently aware of? Are you adding in all of Nominet's IP ranges at the same time?
I would have thought trying to prevent a company visiting your site (and it is incredibly easily got around by any online proxy free to use) would only compound any potential guilt or culpbility? I'm not saying you're in the wrong just it doesn't seem a very reliable prevention.
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