Installed KDE but cant remove it now
Log in on your KDE session, then
- Restore the default login manager of Ubuntu, GDM.
- Restore the Plymouth screen, i.e., the screen with the Ubuntu logo you see during start up.
- Restart the computer. On the login screen, change your session to Ubuntu.
That will bring you back to the default Ubuntu desktop. From there, you can remove kde-full and any KDE applications you do not anymore need.
In removing kde-full, you are removing just one tiny metapackage, i.e. a package that lists all packages deemed a part of the full KDE desktop. If you did not take note of all packages that were added when you installed the metapackage, there is no way to automatically remove these specific packages again.
You can safely remove standard applications of the KDE desktop such as Konsole, Dolphin, etc. At a more advanced level, you could manually inspect installed packages, and check carefully before confirming the removal: if removing a package would also remove the ubuntu-desktop metapackage, or a whole bunch of other packages, then you know the package is also needed for other parts of the system and should not be removed. When finished, run a sudo apt autoremove. This will remove any remaining packages that are not anymore needed because you removed programs that used them.