Use a junior hacksaw.
However you might find it keeps slipping. The way to avoid it slipping is grip the area around the olive with pliers. Let the hacksaw glide next to the pliers, using the pliers as a guide. Once you have cut about halfway through the olive, using a thin 5 or 5.5. mm flat head screwdriver, push it into the grove you have just cut, and twist. It will snap the olive and easily remove it.
good luck.
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