Banning rap shows wouldn't do anything to stop crime as a whole. It would however prevent gatherings that would lead to outbursts of criminal behavior. In order for this to be done, there would have to be documentation and empirical evidence that would show that there is a direct corelation to the rap shows and a spike in criminal activity. Back in my home town there was a concert venue that catered toward the rap scene. You could tell there were always spikes in police calls for service when they had a show. So what the city did was keep records of what happened and then declared the club a public nuisance and shut it down.
There will be one snag though, how will you be able to say one show is ok to have and another is not? Has the Sheriff ever been to a heavy metal or some country shows? Theres plenty of violence at both. I think that there might be an issue if you start black listing some types of "artists" and not others. I can see it now Rev. Al Sharpton marching down LV Blvd. screaming about this issue.
I think the best way to ensure rappers won't play in Vegas is to go after the venues that bring the acts in. If they try to put an act on at a casino, the gaming comission will put the heat on. Then if they try to put on an act at another venue, just bring code enforcement in and liquor control in on it. Make it so hard for rappers to do business here and they will keep on trucking.
However this will set a precedent, once they figure out its ok to chase off rappers, who's next?