Perhaps educating them in martial arts?
I am not sure what to suggest. Baseball bats, cricket bats, golf clubs...for that matter, a broom stick or inch diameter or larger dowel rod might work. The pepper spray is good too. Tasers might work, but you still have to aim them and have the presence of mind to work within the maxium effective distance of it.
In the United States, most of our one on one confrontations happen within 7 yards (a bit less than 7 meters). Someone with a knife that close can cover that distance in less than two seconds, making a firearm almost as useless if you don't know how to defend against the attacker while moving the firearm into a useful position.
I might suggest using a staff or club weapon where you might be able to extend the defender's reach over a knife-armed attacker. An attacker high on drugs may not be stopped by pepper spray and might be all the more dangerous. Tasers may not properly take down someone, and again, requires some basic aiming skills.
You might want to have a silent alarm system to alert you in your office if there is trouble up front or in another part of your law offices. At least you might get a bit more warning for you to act. This might reduce your reaction time to the threat.