Legal enforcement of bans on goods can reduce the size of the black market but lead to greater violence by increasing revenue in the illicit market. However, the link between enforcement and violence is not as simple as is suggested by the textbook model, even for a competitive market. Nevertheless, under plausible assumptions more enforcement on trafficking in the illicit good leads to more violence.
Prieger, James E. and Kulick, Jonathan D., "Unintended Consequences of Enforcement in Illicit Markets" (2014). Pepperdine University, School of Public Policy Working Papers. Paper 51.