How can I remove the Plasma-X11 session from my Ubuntu system?

You might want to try updating your session settings manually. Typically, you can find the session files in the `/usr/share/xsessions/` directory. You could delete the `plasmax11.desktop` file from there. Just make sure to back things up before you change or remove anything! Also, if you use terminal commands, double-check that you're using the correct paths.

If you're struggling with the command line to remove it, another approach is to check the session chooser under your display manager settings. You can often select which sessions to keep active. If that doesn't help, consider booting into recovery mode and check if there's an option to remove the X11 session from there.