We've seen plenty of cases of SEOs threatening Googlers but not Googlers threatening SEOs.
A Google Web Search Help thread has a webmasters who seems clearly confused saying Google called to threaten him to pay them in order to stay included in Google.
He said he received a "phone call from Google stating that despite mailing me letters (that I never got ) and many phone calls to my home regarding my account stating that if I did not respond to this last time my Google account will be permanently shut down."
When asked about what service this was about he said, "a submission to Google."
Seems like he paid a submission company or didn't pay a submission company, to submit his site to Google and they are not threatening to take over his Google account - maybe his Webmaster Tools account - if he doesn't pay.
It is clear that he doesn't understand what he signed up for. But it is also clear that Google wouldn't be calling and threatening him about this or even about if he bought AdWords. Google, from what I am told, farms out collections on collection agencies.
Have you ever seen a case of Google threatening a webmaster outright?
Forum discussion at Google Web Search Help.