I'm a (more or less) happy KDE user, ever since the KDE 3 days. Before that, I used fvwm2 for a long time, but that is a different story. It also happens that I never really reinstalled my home directory - the oldest files I can find are from 1997, and that is pretty much when I switched from an old Slackware system with self-compiled updates, to something with a more modern distribution. That means, that all the time from 1997 to today, I carry the same /home/ads across my computers. The home directory grew from a few MB to 133 GB today (maybe I should clean it up, but then again it's cheaper to buy a bigger harddisk).
It also means, that I never deleted my KDE config, even when upgrading to KDE 4 or Plasma.
After reading through some bug reports, because of an error with my dual-monitor setup, I frequently found the hint to do a fresh reinstall and delete the old account. Now, with 183 dot files and directories in my home directory, I surely do not want to do a fresh installation. There is too much stuff I'm still using and which makes my life easier every single day. Granted there is also stuff which is no longer in use, but why spend time on this? Therefore I had to figure out how to reset the KDE config without deleting everything else.
First problem: you can't delete the configs while KDE is still running. Therefore: log out, and use a simple terminal login (Ctrl+Alt+F1) to login into your account. Another way would be to create a second account, and use this one to delete the config files in your regular account.
Second problem: what if something goes wrong? Create a backup of your system, and store it somewhere outside where you can still access if, even if your KDE does no longer work. Personally, i also created copies of all the config files I deleted, and stored them into a separate folder ... just in case.
After searching for a while with different filename patterns, here is the list of files and directories I came up with. Also had to try it 2 or 3 times, because I forgot something during the first attempt.
rm -rf .kde/ rm -f .kderc rm -rf .config/*plasma* rm -rf .config/*kde* rm -rf .config/*kwin* rm -rf .local/share/kded5 rm -rf .config/session/kwin_* rm -rf .gtkrc-2.0* rm -rf .cache/upstart/startkde.log* rm -f .xsession-errors
After deleting all the files and directories, log out from the terminal, and login into your KDE again. You will find a fresh and new KDE installation, with all your files still in your home directory.
This left important settings like kdewallet (all the WLAN passwords) in place, but did reset all my regular KDE settings. The hard part then was to go over the Settings again, and configure everything the way I like it. Plus re-create all the entries in my task bar.