Don't believe there is a way to report...

  (Edited: )

...but you have to realize that some% ~50% of the people (completely made up statistic) messaging you to be friends after a game are going to be hostile, whether you won or lost.  I can usually tell by how the game played out, what emotes were used, etc., and often I don't accept requests.  One way to stave this off that I have noticed is IF you accept a friend request from someone, immediately (and I mean immediately) message them first with "gg really wp" or something very polite.  This will almost always stop an impolite person in their tracks.  I mean it's almost impossible to be hostile to that (not 100% of course, but really, you have to be a real douche to BM after someone is extremely polite to you, and I don't think "most" people are, even if they act that way in a game)  That said, I don't typically accept friend requests much anymore for those very reasons, and I have enough friends on my bnet tbh, but that's just my preference.  If you are playing a "cheap" deck (again I put quotes on this because I don't believe ANY deck is cheap personally, but the community as a whole does--any hunter, mill/fatigue decks, zoo, etc.) of course expect more BM from friend requests, that's just a given.  So yeah, immediately respond to request you want to accept with politeness whether a win or loss, play control, don't BM emote/make BM plays, and most friend requests you'll get will probably be genuine.

ps I'm not suggesting you have to play control to be considered "good" or anything--I play aggro decks ALL the time--but I definitely notice I get (and give) more BM to/from very aggro decks.