df hangs when there is a bad mount point

> When one mount point is in trouble, the df -k command
> hangs there forever. Is there a way to stop/terminate it?

What version are you using?
In any case, you'll probably solve the problem
by switching to a newer version, since a problem
like what you describe was fixed about three years ago.

The latest is here:

  ftp://alpha.gnu.org/gnu/coreutils/coreutils-4.5.7.tar.bz2
  (coreutils is the union of fileutils, textutils, and sh-utils)

If your problem persists even with the latest release,
please include the precise command that you used and show
what (if any) output was produced.

Thanks for the report.