I have an item in /usr/share/xdg/autostart/ but I want to disable it from starting, I cannot do this without removing the application. It would be nice to be able to disable items in there from starting. It would require the user to elevate similar to the SDDM KCM. The KCM could then create appname.desktop.disabled in there which would prevent it from starting with Plasma. Changes to the .desktop file itself would just be overwritten on next update so altering that wouldn't work.
If you only want to disable it for the current user, not all users, then this doesn't need root permission. The desktop file in /usr/share/xdg/autostart can be copied to ~/.config/autostart, and modified to contain "Hidden=true". This should prevent it from autostarting.
*** This bug has been marked as a duplicate of bug 274920 ***