I´m making a game that simulates an industry of pan, and one of the process is Painting.
What I want to do is to let the player paint the pan, but i don´t want it to
Your essentially dealing with a collision detection problem. From looking at their API you could try something like a for loop with getColorAtPoint and try to determine they have drawn at each pixel.
If all else fails look into collision between the objects the library generates using the .hitTestObject method of an object.
See this: http://sierakowski.eu/list-of-tips/39-collision-detection-methods-hittest-and-hittestobject-alternatives.html
And this to see how someone handles collision with pixels: http://www.emanueleferonato.com/2010/08/05/worms-like-destructible-terrain-in-flash-part-2/