If a user sent abusive language in the game and then unfriended you, you can report them by going to the Social tab in the battle-net client, finding the chat, right-clicking the abusive message, and selecting the report option. (Which reports the message, and by extension the player).
If you suspect a user of cheating or hacking, then you can report it using the instructions found at How to Report Hacks/Exploits/Cheating
And if someone violates forum code of conduct, you can report it by reporting the offending messages. Which, again, reports the message and by extension the user.
There are, as far as I know, no other ways to report users.