Not sure how'd they enforce this to be honest.
The "70 euros spend per head" is a grey area anyway, as the waiting staff aren't concerned with this policy from what I've experienced. ie. If you only spend 50 euros a head, I'm almost certain nobody's gonna pull you up on it. A lot of diners are there just for the meal anyho and have no intention of visiting the club night after.
Once you've finished your meal and paid, you're free to leave at your own discretion.
And of course there's the fact it's very easy to disappear into the club via the restaurant anyway. I'd imagine this would be even moreso the case on a particularly busy night.
I'm not for one minute condoning such actions or suggesting that people should try this, but it seems to me this "rule" is simply in place to put-off potential piss takers as opposed to genuine customers who want to dine there and then hit the club afterwards.