Saw it and file. I forgot to mention, you should stick either a "fence" of tape around the sides and bends of the sight base, or make a "mask" with a thin peice of brass. Just take a thin peice of brass shim stock, and cut a slot big enough to fit over the base. This will offer some protection from mis-strokes of the file.
I don't recommend trying to apply heat to melt the brase holding the base, since it takes so much heat that you usually damage the finish.