One way to balance this might be to implement bounty hunters.
I know I've preyed on the occasional frieghter/Zulu black market squad.
The corporation already sends cruisers after you if you destroy corp stations, and black market sends it's bounty hunters. There could be commonwealth sponsored bounty hunters that come after you if you're seen leaving a Black Market facility.
-They should have a 20%-50% spawn rate (depending on strength)
-They may come after you even if you didn't purchase anything from the station (they just assume you're loaded up with illegals)
-Commonwealth bounty hunters could be in in a variety of ships since they're civvies; eg. sapphire, wolfen, even reimplement Mikeno destroyers in earlier systems.
-They should send you a message (similar to We FoLoW the StROng) along the lines of "Surrender your illegal cargo."
At which point, they could either just keep shooting at you, or you might have the option of holding ".", being boarded, and let on your way sans drugs.
Destroying these bounty hunters should NOT be illegal because they'll be aggressors and they don't have proof of your illegal activity. Also, it'd unbalance in the wrong direction if you got arrested for that.

Also, they'd have to be coded to give you benefit of the doubt when you're loaded up with nice, profitable slaves.
