Mmmh, it seems that "imclearborder" won't work. However, that thing is going to be the largest connected component of the image. You can detect it using "bwconncomp". Actually, the documentation for "bwconncomp" has the exact example that you need:
%Find the number of connected components in the image.
CC = bwconncomp(BW)
%Determine which is the largest component in the image and erase it (set all the pixels to 0).
numPixels = cellfun(@numel,CC.PixelIdxList);
[biggest,idx] = max(numPixels);
BW(CC.PixelIdxList{idx}) = 0;
%Display the image, noting that the largest component happens to be the two consecutive f's in the word different.
figure
imshow(BW)
