So that everyone else knows: Many distros start a package update check when you log in, though the icon doesn't appear until just before the notification that say "so many packages are available for update...". While this is running, it's using yum and creates a lock file (usually at /var/run.yum.pid). Removing this file will allow you to use yum from the command line, though that's somewhat unsafe (in case you decide to hit the update dialog and start 2 concurrent updates).
To stop this from running automatically every time you log in (under Gnome anyway), System > Preferences > More > Sessions > Startup Programs and uncheck or disable it from there.
Also, to kill a process by name, you use "killall" as "kill" is used for process IDs (ps -A | grep puplet)