how to make tramp forget a host?

[Sam Steingold]

Hi,
I have a _recurrent_ problem that I would like to avoid:
When a host becomes unavailable, tramp _sometimes_ hijacks Emacs
preventing all progress, including normal quit.

When I use VPN, I cannot use IMAP and sendmail, so to use gnus or report
bugs I have to get off the VPN.
However, I have many open files residing on the host cloud-desktop,
and I do _not_ want to kill those buffers (the buffers are saved though).

I also have an open file on fencepost (available both with and without
VPN).

Now, when I get off the VPN, _any_ action triggers the tramp error:
I see the buffer "*tramp/scp: cloud-desktop*" saying "cannot find host
or nodename cloud-desktop" (or something similar) and a message in
*Messages* suggesting `tramp-cleanup-this-connection`.
The only way to stop the incessant beeping is C-g, after which the
*tramp/scp* buffer is no long there!
Even when I do M-x tramp-cleanup-all-connections (!) and
_kill_ all buffers on `cloud-desktop', and even do

--8<---------------cut here---------------start------------->8---
(dolist (b (buffer-list))
  (when (string-match "cloud-desktop" (buffer-name b))
    (kill-buffer b)))
--8<---------------cut here---------------end--------------->8---

(which kills hidden buffers too)
I _still_ get the tramp error about `cloud-desktop' - even on C-x C-c.

Now, I am pretty sure this sounds preposterous - you would have fixed
this if you ever encountered this, so all I am asking is _suggestions_
on how to _debug_ this.

Specifically, if I killed all the cloud-desktop buffers, how does Tramp
_still_ know about cloud-desktop? Is there a variable which contains all
connections ever made?

How do I get the *Backtrace* for the "try tramp-cleanup-this-connection"
error? (I am not getting it despite `debug-on-error` being _always_ t)

Thanks
-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.2113
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