The belt was squeaky about 3 months ago, we tried the spray, and then brought it to be looked at. They said the pulleys were fine, but the belt was worn and they replaced it. The squeaking stopped for maybe a month, then returned. It squeaks when you first start the car and its in an idle, then you can hear a very faint squeak when its put in gear, then after its been driven for a while there's no squeak at all when its in neutral.
Never never ever use any kind of dressing or coating on a flat serpentine belt. When you had the belt replaced, did you tell the mechanic there was a foreign substance on it? If not he would have no way of knowing he needed to scrub all the pulleys. That stuff will get transferred to the new belt and make it squeal too.
Assuming the belt is being held tight by the tensioner and none of the pulleys are turned or tipped due to a worn bearing, you can try spraying it with engine degreaser while it's running, then washing it off with water from a garden hose. If that doesn't help, replace the belt again and wash the pulleys before you install it.