Easy way:
B = zeros(size(A), 'like', A);
B(350:400,20:35) = A(350:400,20:35);
Now B is A with all the other elements zeroed.
A different approach:
mask = true(size(A));
mask(350:400,20:35) = false;
A(mask) = 0;
A third approach:
A(1:349,:) = 0; A(401:end,:) = 0;
A(:,1:19) = 0; A(:,36:end) = 0;
