Answer to your question
To flag a user, just flag one of their posts for ♦ attention ("other") and, in the box, mention "Please check the profile of this user, if contains inappropriate content". Usually useful for NFSW gravatars, etc.
Answer to your situation
I heard somewhere that the bio is fair game--you can post nearly anything there, including expletives. Racism, etc may not be tolerated, though. I've seen profiles basically insulting the whole audience of the site--though brought to the attention of the mods, they may not change it (mods can edit bios).
The bio is only visible as a tiny bunch of text when you mouse over the gravatar (for 1k+ users only). I don't think that there ought to be any NFSW issues with tiny-font expletives.
In this particular case, the bio is just repeating a well-worn joke. Harmless, really.