In spite of perfect detections of fake clicks is impossible, at lest trying to detect and filter them is a nice problem.

I don't expect advanced solutions against your site. It's not a bank, nobody would get a huge profit with fake clicks. You need to fight against script kiddies, not specialized hardware.

So I guess some easy heuristic to detect people that clicks too fast or do not sleep for a week will be enough.

Indeed. Thanks for advice. I will implement it. I will first work on a throtting system will and a way to detect programmed clicks. This should already drastically reduce spam.

Those two Spanish accounts look very suspicious...

Indeed.