Sounds like a pretty unfortunate incident, for the best part, it elucidates the flaws in our 'lenient' legal system.
This individual assaulted you, therefore you should report it to the police and pursue the matter. He may threaten to start a war,
but a few visits from the police usually calm such people down. While he may appear to have scant regard for the law, being a dealer
and all, unwanted police attention is the last thing he needs. The worst thing you can do is let him believe that he has yourself and other
office staff fearful.
The new security measures are a good step, beyond that, make sure you report the matter to the police, particular any escalations. Try to record
any future conversations on your phone and finally, have the vehicle towed for being parked on private property against your will.
Yes the law sucks, but if you learn to use it to your advantage you can at least get by....