Someone published our Google Maps APIKey - how can I force him to take it out?

We have an open-source CMS which has a demo-google maps API key to get started.

Someone took this api key and published it in unrelated source code - so we got the warning from google.

Since the CMS is installed on thousands of third party systems we cannot just change it.

I already tried to contact the person but never got a response since February. Stolen Google API Key · Issue #1 · imjvdn/aps · GitHub

How can I escalate this and force the removal?