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Here's a previous thread [webmasterworld.com] which includes some email from GoTo customer services.
It's clearly essential that GoTo and other PPC engines have effective safeguards against spurious clicks. Their very existence depends on it.
on the whole i agree with tedster. i've checked logs in the past and compared "total" clicks with "billed" clicks, and its obvious that GoTo is throwing out click and not charging us for them.
that said, i've still seen potential abuse in the form of "strange" URLs like the one you mention, generating repeat traffic. but often, upon further exploration, these URLs are often believable. sometimes the keyword will be X and the site will www.X.com, offering a list of links from GoTo related to X. if www.X.com ranks high in many other search engines, its possible that it will get a decent amount of traffic, and pass some of that on to you.
re: the security device you mention, probably not a bad idea. best bet would be to figure a way to seperate out all questionable clicks, so that you could review these on your own, and perhaps turn them over to GoTo for their own review. if you clearly identify abuse from a partner or affiliate, they will promptly take action to shut that affiliate down. as tedster says, its crucial to their credibility.
seoboy