My preference would be a press to remove them, that said, We often left the nut on the cotter loose,then banged against that. I would highly recommend supporting the crank arm as you are driving the spindle into the bearings and may cause damage. Hammers I have used besides the ball peen are Brass and my favorite,a double ended hammer with a plastic end and a rubber end. Wood laid up against the part also makes a protective cushion, in fact I use that trick often. BOB FREITAS who doesnt have many bikes with cottered cranks in MILL VALLEY,CA USA
PS what we did back in the day was one thing as we had boxes of spindles and cups for cottered cranks these days that stuff is pretty rare so act accordingly. Also apologies to those customers who I did not take extreme care while working on their $49 bikes