This is first available during the 'the way of the flesh'. This is the prostituion quest line started by The Voice.
You will be tasked with destroying Merles racket to the guns of $15,000. After freeing the girls, you will have two enforcer targets to kill.
You must kill both of these before returning to The Voice. It is possible to finish this questline by only killing one if you meet the target earlier.
This works on any other mission where your tasked with destroying a rackets enforcers.