Merchants weapons

[Condottiere]

I think they have to be licensed, in order to dock at an Imperium starport.

 

[MonsterX]

It is possible to check if missiles are radioactive. You could keep them in a lead-lined magazine (a good idea anyways), but this is likely to arouse suspicion. If they're stored next to the fusion plant, the radiation might be hard to differentiate. But storing your missiles next to the fusion plant instead of the turret is kind of suspicious in itself. If customs want to check, it can. Penalties for smuggling nukes might be a little harsh, given that this is likely to be strongly associated with terrorism or insurrection.

Not only ships' weapons but the ship itself is a WMD. Ram it into a city after accelerating 6Gs for a few days and you'll get a big crater. Planets will have at least minimal defenses against this if they can afford them - even if they have to important them from expensive high tech planets. I assume that basically every planet with any kind of decent economy and or infrastructure is protected well enough to prevent a typical PC ship from succeeding at doing this kind of thing.

 

[Firedrake]

REGULAR ship weapons are WMDs, even lasers, against civilian targets.

Depending on which version of M-drive you're using, the ship on its own is also a WMD (either as a high sublight kinetic weapon or by hovering over a target). But if you follow that chain of reasoning, you don't get independent merchants, you get starship captains picked and trained like ballistic missile submarine captains.

 

[Condottiere]

Automated mass driver bay, to place a couple of boulders in the way.