I wouldn't do a thing. A few years ago I had some 150 clicks from a single IP come in (who knows why), AdSense rightly just ignored them.

I had a couple of thousand clicks on the day after a troll was banned from our forums, and Google ignored all of them. I didn't move a finger.

I used to have this happen every few months, someone decided to hammer a few hundred clicks across several domains. Google rightly ignored them however they used to leave the invalid clicks there which made the CTR and eCPM look terrible.

I'm not too sure if they delete them nowadays, I have a funny feeling they do since I haven't had this kind of activity for maybe a year therefore if they do delete them before we see them we wouldn't even know.

A plausible explanation as to why some of us sometimes see delayed stats.

I have a stats tracker for my sites and have also seen a number of clicks that I would classify as suspicious and like you have said Adsense has ignored them in my case too.

I just don't remember them being blatant enough to actually send me email

Don't ever respond to messages like that, they will get bored and move on eventually.

Don't reply, but forward it to the computer crime unit of the police. Blackmail per email is illegal too ... He's probably blackmailing others as well.