How to prevent AdWords click fraud

A friend of mine and me are getting into lead generation here in Germany at the moment and things have already worked out very fine for us. We used Google AdWords to direct paid traffic to our landing page, take their phone calls and then successfully sell those leads to businesses all across the country.

Now here's the problem: Our competitors don't really seem to appreciate our efforts. Somebody has started to excessively click our ads using proxy servers and a click bot or something like that, because the clicks come from different IPs every time. BUT they always come within a really short period of time and continue until our daily budget is consumed.

We did get back our money from Google every time until now, but they don't seem to have found a real solution thus far. And our problem is that it always takes some time until the money is refunded. And during that time we are basically paralyzed because our budget is still limited. Click prices are between 25 and 50 $ per click.

Has anyone dealt with this before? What can we do against this kind of click fraud?

Thanks, guys!