I ticked the "automatically show this window" box for showing the display preferences when attaching/detaching an external monitor. However, this has started to annoy me and I don't want this to be shown whenever I unplug the cable. I can't for the life of me figure out how to undo this setting -- how do I do this?

Was this checkbox on a dialog asking "Do you wish to run a configuration tool to adjust the monitor setup?"?

Yes, I believe so, but I don't remember the exact text. I recall that it also mentioned that the tool should be run whenever a monitor was attached or detached.

I can't seem to find the code which triggers the automatic display of the control module, however this is a KDE Daemon module - so the configuration file for "kdedrc" or "kded4rc" is likely the right place to start looking.

You can find this at ~/.kde4/share/config/ or ~/.kde/share/config depending on your distribution.

I don't see anything in ~/.kderc other than font settings. I was already poking around in ~/.kde4/share/config, but there are lots of files there and I'm not sure which one to edit. The krandrrc one seems promising, there is a section that begins:

[Display]
ApplyOnStartup=true
StartupCommands=xrandr --output LVDS-0 --pos 0x0 --mode 1600x900 --refresh 60.0012\nxrandr --output DP-3 --off\nxrandr --output LVDS-0 --primary

Would I set "ApplyOnStartup" to false? I have a feeling that's wrong -- it seems that this would set the xrandr command when I log in. There is also something in the startupconfig script about krandrrc_display_applyonstartup. Could that be it?

The ApplyOnStartup and StartupCommands options are actually related to the applying of your desired settings on login - and are not related unfortunately. Could you get me a screenshot of this dialog (a new user should show it) please?

I've uploaded a screenshot of the dialog here: https://www.dropbox.com/s/fxfzqar249n0g79/monitor-dialog.png

It was the "do not ask again" box which I ticked after clicking "configure", so the monitor configuration tool runs every time now. I with to either always ignore or reset the preference, if possible.

It would seem the dialog you are getting differs from what the code I have here indicates should happen.
However, based upon what I have found, you need to look for either "kdedrc" or "kded4rc" in ~/.kde*/share/config/

Inside that file, find the section "Notification Messages" and remove that as well as all entries below it until the next [] section tag.
Logging out and back in again should then fix this.

Thanks, I finally found it. There is a part of ~/.kde/share/config/kdedrc which reads:

[Notification Messages]
randrmonitorchange=true

Setting this to false fixed it.

Thanks very much for your digging ) I also thought it would be great feature, but it is really annoying.