Most often, Web site owners repeatedly click the ads on their own sites to generate money for themselves. In other cases, advertisers target the ads of their rivals to drain their marketing budgets. I would think that in most cases this would generate only one click per user. At least this is how Yahoo does it. Unless the fraudsters are killing their cookies and then clicking again, I don't see how a single user could rack up multiple clicks.
They use infected computers as Bots to so the clicking for them in a lot of cases.