There's quite a bit of evidence that the exact opposite is true, that violence increases with stronger police enforcement, due to the destabilization of established trafficking lines and the resulting struggle to fill the void.

Here's a study done on the Marijuana trade in Denmark:

http://euc.sagepub.com/content/early/2013/01/10/147737081246...

Mexico is an even better example. Minimum 60,000 dead after Felipe Calderon dramatically increased resistance against the cartels.