remove unwante noise

sir, how can I remove unwanted pixels? here is the photo: http://www.flickr.com/photos/77154085@N06/7354365440/in/photostream

I just the circular road sign only. I did calculate the circularities but it just gives the value of the shape not eliminate the unwanted shape.pls advice.tq

[Tulips]

basically, I used bwareaopen =1000 to remove unwanted noise. however, this 1000 pixel works only for the current image. if the ROI is taken at a certain distance(at long distance , the ROI will be eliminated and separated. then, what shoud I do?