Clearing static nat on Cisco router

I was asked to change a incoming NAT translation on a Cisco router for a customer today - however since this NAT was used to deliver all their internal email, it was never not in use, and I got the standard message below when trying to clear it:


This is a companion discussion topic for the original entry at https://www.funkypenguin.co.nz/note/clearing-static-nat-on-cisco-router/