What is the connection to the exterior downspout like. Internally it can be just cut off but you will need to deal with the other end of it as well or you may cause yourself damp problems.
If the cast iron is now redundant then cut it flat and then chisel it back so it is recessed, then fill it with insulation or some other filling material, or even a brick and then seal it right up with morter.
Ideally it would be chopped and then the whole pipe chiselled right out and then the hole made good with brick cuttings and mortar
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