Making non-circle shaped forms disappear

I've tried to find a way to make disappear a non-circle shaped forms but I didn't succeeded. Could someone suggest a way to do it?

[Sven]

Readers please ignore this question, it is duplicated at:

http://www.mathworks.com/matlabcentral/answers/19507-remove-non-circle-objects-from-image

LG9, to keep "Answers" as a useful tool, please read the "About MATLAB Answers" link - it will tell you how to ask questions with the best chance of getting an answer, and how to accept an answer to make it easier for others to find that answer.