Google AdSense Account Disabled" with balance of $42,000 and monthly revenue of $60,000

Dear folks,

Any help would be appreciated to solve my issue. We run a social network with around 140,000 active members and it’s been running since 2008. The only source of income is adsense. We have used different tools to bring more users and engage more users in our website and we have around $2000 daily revenue. There hasn’t been any click surge or spike or any sudden change in the revenue lately, we just have improved quality of site. We received the notice “Google AdSense Account Disabled” with this content:

“After reviewing our records, we've determined that your AdSense account poses a risk of generating invalid activity… “

We do have long run Analytics report, our analytic is linked to ad sense so it should be easy to show there hasn’t been any click fraud at our end. Why would we risk a large scale business for couple of fraud clicks! Our current account balance is more than $40K and Google says it will be returned back to advertisers! We have spent almost this much to deliver traffic, pay for engineers and servers…. What is the best way of approaching and what are convincing evidences to show our activities are all based on good faith and there is no intension of getting non legitimate clicks.

Any help would be appreciated, this incident has put a big question mark on job status of 5 hard working engineers in such a tough economy and future of a project that has been developed out of enthusiasm and passion during last 3 years.

Thanks,
Mike