A knockout block for removing a reluctant nib from a section is an essential pen repair tool. It consists of a solid block with a number of holes of differing sizes to accommodate a range of nib sizes.
A range of punch sizes to fit the section bore is also needed. The picture shows a very substantial and versatile block which will last a lifetime and more. As you are only ever going to buy one of these it is best to buy a good one.
The photo is a block supplied by The Pendragons at Thependragons - Vintage fountain pen sacs, fountain pen parts, tools and repair kits (vintagepensacsandparts.com)
It is first class and costs £29.