Where I work we have gotten proxies that cannot be deleted with any export to autocad, or any creative method such as xref into a new template and bind. When this happens it is usually from the Network130 or some other smart pipe or pressure pipe proxy, dropped off by a Civil 3d user of a more current year. That said, the solution I found is to either enable your software with all the years leading up to the current year, or finding that Zombie Killer LSP, and use that to run the command in the culprit drawing, and it finds those proxies I mentioned and deletes them all. Works like a charm for me since I found zombie killer nearly 5 years ago or more.
