Talk to your boss again. You boss must state this is law from now on. If he doesn’t want constant fights, make up sufficiently hard rules for exceptions that people don’t take them lightheartedly.
If this goes on for to long, people will get accustomed to it, and it will be harder and harder to change... Maybe the boss has to involve his boss.
You alone can't do much without backing. You could try to persuade your coworkers that following process is good for their CV, and the ability to politely tell no will further their career anywhere. This is true, and also hard to sell...
