I don't know if this is the best way. But have a look in /home/username/.kde/share/config/session
you should see a bunch of files there. Look for files starting with the name of the app you don't want autostarting and move them to a backup. If everything goes okay delete them.
/edit also, have a look in control center under "kde componants" and then "session manager" and should be an option to not restore session. Is on mine but my distro isn't mutalated by red hat