That is why I would use a impact socket, putting the cv nut flush and hitting shaft with the hammer will mess up the shaft threads and or nut.
Learning experience for next time.
Use a impact socket, there beefier and can handle the abuse your going to give it, when you hammer away, I use a 4lb sledge hammer for stuck cv shafts
Make sure the impact socket is smaller in diameter then cv shaft end, cv nut flush to prevent thread damage , just in case.
Sounds like you got one of those pain in the ass cv shafts. PB blast it really good on the back side , if you can get it to go in