One typical and simple way to do this is to use imopen function. Here is an example.
% Reading your image
I = imread('123.JPG');
I = rgb2gray(I);
BW = imbinarize(I);
% Apply imopen function
se = strel('disk',3);
BW2 = imopen(BW, se);
% Show the result
imshowpair(BW,BW2,'montage')
