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Be very carefull trying to enlarge a bore in a plastic handle with drill bits,is a recipe for disaster,is the only time I would use a Dremel with a small sanding drum

Silvertip Standard Knots : Fountain Pen parts - NIBS, - Wholesale Fountain Pen Ink - Golden Nib AZ

Wrap the knot heavily in masking tape,clamp it in a vice, than pull hard,if that will not work, cut it off close to the handle.
Drill a series of holes in the knot and pull the hairs out with pliers.