OK Shef - take a deep breath.
1 Dick went to management who declined to do anything. This gives you at least some power. Write to management. In the letter state that you are fearful of the repocussions from the druggies. Point out that Dick went to management not the cops and as such you expect management to keep their staff under control. ANY act of aggression or threat will be taken to the police, together with a copy of 'this' letter letting them know that management knew.
A copy of 'this' letter will also be sent to your lawyer, OSH and any other bloody agency you can find together with Campbell Live, 20/20 and 60 Minutes.
Let them know you will anihilate them.
2 Set up some motion senitive web cams. At least one more if you can.
3 Don't count on oven cleaner. You have to get to the can, then have it facing the right direction to use it, as well as being close enough to use it.
My cunning solution is to get some high powered water pistols and fill them with amonia. When you are asked why they were filled with amonia, you say they had developed mould and you were cleaning them but forgot in the heat of the moment. Every one is going to know that is bs, but they can't prove it is bs.
I don't think they are going to get close enough for any hand to hand stuff anyway. They might yell some stuff or use some scare tactics, but they know Dick didn't go to the cops or get them fired. They know he left and as such he is next to no threat to them anyway. Reinforce that with management and hold them accountable.
Hugs