It's supposed to be vague. We can't support new member hazing, since any member that leaves the community as a result is, in a sense, a loss to the site. At the same time, there's really no fun in strictly moderated spam.
As a general rule, I infract when the user being flamed reports the post with a complaint. If you're insulting someone who knows it's a joke and who you know won't take any real offense, then have at it. Otherwise, any non-trivial flaming - in particular, anything done with the expectation of causing offense - is infractable.