You can use the coordinates to get slope and intercept of the dividing line. From there you could work row by row zeroing out the pixels to the right of where the slope meets the row; or you could vectorize it all using ndgrid or meshgrid on indexing matrices...
[Y, X] = ndgrid(1:size(YourIm,1), 1:size(YourIm,2));
NewIm = YourIm;
NewIm(repmat(Y < m*X+b,1,1,3)) = 0; %the repmat duplicates the 2D mask into all bit planes
If you have a sufficiently new MATLAB, R2016b or later, you can do it without the ndgrid:
Y = (1:size(YourIm,1)).'; %column vector
X = 1 : size(YourIm,2)); %row vector
NewIm( repmat(Y < m*X+b, 1, 1, 3) ) = 0; %the repmat duplicates the 2D mask into all bit planes
