In my .emacs file, I have
(setq transient-mark-mode nil)
and yet transient-mark-mode is activated when I start up emacs. Calling transient-mark-mode gives the message 'transient-mark-mode disabled', but C-space still starts highlighting a region. Is there a way to turn this off?
You might have cua-mode enabled as well which has similar region highlighting to transient-mark-mode. To test this just M-x cua-mode and see if it disables the highlighting.
From C-h f cua-mode help:
CUA mode manages Transient Mark mode internally. Trying to disable Transient Mark mode while CUA mode is enabled does not work; if you only want to highlight the region when it is selected using a shifted movement key, set `cua-highlight-region-shift-only'.