Need suggestions for blanking hole in chrome taps

Could you not just cut off the knob with an inch or so of rod underneath an stick it into place in the hole?

That's my fall-back plan, but I'd like something more 'flush' if at all possible - all the better for management approval!!

Why not use the sort of plug part that is in fact a chrome knobbed threadaded thing, that the plug chain attaches too?

Drill and tap your tap for it.

The best idea is what someone else suggested ( amended slightly )Just leave the rod in situ but without it being connected to anything .That will give you a far neater finish than any bodge will. Chrome nails.Araldite ,chrome nobbed threaded thing ...FFS ....Don't even cut it shorter .who the Hell is going to be peering under your w/h basin? ...Folk living in the house will know it is just cosmetic and not use it .Thats what I did .

Dead right, pop up wastes are a right PITA, always drip/leak, pig to adjust

As others have suggested simply cut off the rod & leave the knob in place - the tap was meant to look like that!