You say "He boxed me in in the kitchen and said I deserved to be "punished" for a bug found in my code the day before. He then claimed I'd "enjoy being spanked". "
This isn't just harrassment, this is assault. According to what you say, you had to be in reasonable fear of physical force against you. And that is assault, even if the physical force didn't happen. It's criminal. People can go to jail for this.
My advice to the boss: If this is a person that you call a friend, then you really need to be more careful about your relationships. This man is not anyone's friend, he is an animal.
Write down what happened, and what you reasonably feared would happen. Take this to your boss. Tell him that there is no "flirting" going on whatsoever, but that you have been assaulted. If your boss doesn't think it's assault, tell him that the police will think otherwise. If he doesn't want to do anything about it, tell him that he as your boss is personally responsible for the safety of the workplace. That by his inaction he is making this a very hostile workplace.
On the other hand, if he is convinced that this is just harmless flirting, then he won't mind if you tell every male colleague about how this man has been acting. After all, it's just flirting.
PS. After your final update: With what I know now, your boss has acted truly irresponsible. So he had a friend who has been fired for sexual harrassment more than once and has difficulty finding a job. In that situation, if the boss decides to help his friend out and hire him, then it is absolutely essential that he tells the friend that such behaviour will be totally unacceptable at his company, and it is essential that he acts in the most forceful way possible if there are any complaints about him.
When you complained the first time, the boss should have talked to his friend and made it clear to him that this is his last chance. He should have told you that this behaviour would never happen again. Or at most once, and that you should tell the boss immediately if anything happens again.
What he did instead, telling you that this was "just flirting", was absolutely and inexcusable wrong. What he did would have been bad and wrong if he hadn't believed you, but he knew or should have known that you were telling the truth.