Just don't go to La Rambla at night, or briefly pass through to go to the Barrio Gòtico clubs (an area near La rambla). A pair of hookers ever 5 metres. We got stopped there drunk and they started groping my friend. Be authorative and they'll fuck off to someone else.
Check out a club called Razz (Matazz), an old warehouse type of club with excellent parties, but don't get there before 4 am. Remember, Spain has dinner at 11 pm, hits the bars around 1 and goes clubbing aroud 4-5. Enjoy!