What is the correct way to write the interjection "ha ha?"
ODO has “ha ha”. Normal rules of capitalization, punctuation and the like apply, so a complete interjection might be written “Ha ha ha!”
As an onomatopaeic word, it would be possible to join the individual sounds together to get hahaha but this is more suited to less formal environments.
Don’t hyphenate it, because a ha-ha is something rather different.