fill inside a specific boundary in bw image using poly2mask is not accurate

I have written the following code to delete a specific boundary using poly2mask:

function newBW = fillBoundery(B,bounderyNum,bw)
    newBW = bw;
    [m,n] = size(bw);
     x = B{bounderyNum,1}(:,2);
     y = B{bounderyNum,1}(:,1);
     reg2del = poly2mask(x, y, m, n);
     a = newBW(reg2del);
     newBW(reg2del) = deal( ~a(1,1) );
end

where bw is the bw image, B is a boundary cell produced with bwboundaries, and bounderyNum is the index of the boundary in B that we want to fill.

this code works with one problem: the boundary is not fully filled. for example I have this bw:

and when I run

newBW = fillBoundery2(B,2,BW);

( 2 is the boundary of the smaller object) I get this:

how do I get a more accurate selection of the region inside a boundary, using this method or a different one?

[Guillaume]

Are you trying to remove the boundary around the object or do you want to remove the whole object?