How they regulate the difficulty, I mean, what prevents you from mining ?

It is adjusted every 2016 blocks based on how quickly the last 2015 blocks were found. For more details, you might want to read these pages:

https://en.bitcoin.it/wiki/Target
https://en.bitcoin.it/wiki/Difficulty
https://en.bitcoin.it/wiki/Mining

This is for bitcoin, every altcoin can change this and the algorithm which is used to adapt the difficulty.