They could have simply switched-off the revenue counter then send you an email notice.
Their Adsense Panel doesn't even tell a publisher about detected invalid clicks (or am i missing it?). I'm not sure why they don't just display a huge alert banner in red fonts on your Adsense Panel saying "Invalid Clicks Detected", so you could have fixed the issue right away.
It would be nice of them .. of course if someone was
deliberately generating invalid clicks themselves, then they'd know that Google was wise to them ...
They do detect them a lot of the time(and remove any revenue they generated) - problem is that they can't know for sure why it happened. How do they know you didn't pay someone to pad yur earnings a little?
It's a good idea to stay on top of your logs and stats for everything - doesn't take all day, just checking everything once a day will catch a lot of things. You can also set up cron scripts to alert you, if you're ambitious.
If I see anything out of the ordinary in traffic, earnings or clicks anywhere (not just Adsense-related) I look into it. I had a good scare last year when one of my pages on a "dormant" site all of a sudden started getting a lot of traffic and racking up clicks - turned out there was a new and aggressive marketing campaign for a small product I'd written up a couple years ago! That was a nice surprise, other times I've had to ban IP's. (Most of my pages are "noarchive" so caching isn't an issue.)
BTW, the "report invalid clicks" form is somewhere in Adsense help.
PITA, but that's life - good luck!