I decided to do a quick check-up on the rules for this. As you as a Gangster have no reason to be in the PD except raiding, it's always a valid KOS in there (or so I assume, anyways).
Therefore the more interesting rule is this one:
4.14 Purchasing Properties - You may only claim multiple rooms/buildings if you have enough people basing with you to make it worthwhile the owner of the house is the one that owns the front door. When taking over a district you must have one person in each house before blocking the district off. You CAN'T block a district that has a NPC inside.
Players should have full access to NPCs and all of their functions and walk after business is done, no exceptions.
So if you look at first glance, Pro could be right. However, as there was nobody there to do business with the NPC in the first place, I don't see how you were obstructing anyone.