Go to Google forum ask the question and seek help there.
Usually Google approach for this case is they can easily identify repeating clicks but they will not help small publishers because they always think it's not Google job to make the thing easy for publishers and it's publisher responsibility to take care about it.(Just like when you paid someone do something you don't expect you going to help him.) For them it is much easier to disable problematic publisher then help them solve it since most of their system is automated and new small publsihers are keep joining everyday. They will expect you to take down adsense completely on the web and wait till you solve with the person.
Suggestion:
1.Take down adsense for week, inform Google to see whether they got reply you or give any suggestion.
2.Find open source script for ads serving which prevent ads appear on same IP, cookies and few more option to add it urself.
3.If unlucky still doesn't stopped him then just go adsense appeal. Usually they value ur content and stuffs.
4.Denied appeal, move on to other ads.
Another solution I would advise(If no reply from Google for long)
Find a big publisher website that serve adsense and copy their adsense code down put on your web. Reply with the jackass say click as much as he like because you will not get banned coz you know some secret. When you free you make a screen send him say thanks him you had make $100, ask him do more. After he click for long and see the ads still keep appear you make big buck he will give up. Google usually don't ban big publisher even if they ban, you not care it

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If you want really want to have Google support you try be more active in their new products. Occasionally I got invited to their activities in Singapore like 1 to 1 suggestion on my web SEO/ webminars/ conference. There you ask question will get more attention.