I've chosen to play another scheme so I am not here to complaint but just to know, the situation was:

The opponent cowed 3 times almost consecutively in a wxw, missing 1 wall and attack. I still was in advantage on him.

One of this time he has also not skipped the turn.
One of this time he has shot himself.

After the 3-4 cows, it was enough for me, and I told him not to play and so we both chose to quit the game and play roper.

But I wanted to know what I had to do, just to know for another time...

In this case Can I take a free win? There is a rule about this? I don't think is fair cowing 3 times allowing you to attack, without missing the wall he could not attack, he was slow. oO

if it's done on purpose, you're probably safe to quit and report if you truly want. but it sounds more like a misunderstanding of the rules if someone breaks them 3 times in a row, so you could be helpful and have a teaching moment if you're inclined.