One way to handle these things is to make them go through your boss for everything.

I had this kind of problem where I used to work. I was one of the managers in question and we told our staff to do exactly that. What happened was we had two types of staff. Those that were too timid to say no and those that "liked to impress" and would say yes to everything anyway. It was hard work trying to beat off other department heads to try and get control of the dapartment. We would have them activly sneeking in when you were out of the room in order to give your staff work.

---If it doesn't fit use a bigger hammer