How about applying median filter?
The following is an example:
% Read the image and convert it to grya-scale
I = imread('gray.png');
Igray = rgb2gray(I);
% Apply median filter to the image
Igray2 = medfilt2(Igray,[7 7]);
% Show the result
figure
imshowpair(Igray,Igray2,'montage')
