Regarding your new, edited question, just figure out the rows and columns to set to zero. If you use fft2(), the origin is at the 4 corners. You'd have to erase each corner one at a time. Or else use fftshift() to move the origin to the middle, then erase, and then use ifftshift() to shift the origin back to the corners before caling ifft2. Here's a start
[rows, columns] = size(grayImage);
ft = fft2(grayImage);
ft = fftshift(ft);
row1 = rows/2 - 15;
row2 = row2 + 31;
col1 = ....
col2 = ....
ft(row1:row2, col1:col2) = 0;
ft = ifftshift(ft);
grayImage = real(ifft2(ft));
imshow(grayImage, [])
