If you don't have gconf-editor, you can install it with yum (yum install gconf-editor).
mybox $ sudo gconf-editor
Then expand apps->gnome-screensaver
Change the appropriate settings and then right click on them. You can select “Set as Mandatory” to lock this setting for all users.
If you have many systems you need to lock down you can use the command line tool called gconftool-2. Below is an example of the command line usage to change the screen saver settings to for screen lock on users after 15 minutes of inactivity.
mybox $ gconftool-2 --direct \
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \
--type bool \
--set /apps/gnome-screensaver/idle_activation_enabled true
mybox $ gconftool-2 --direct \
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \
--type bool \
--set /apps/gnome-screensaver/lock_enabled true
mybox $ gconftool-2 --direct \
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \
--type string \
--set /apps/gnome-screensaver/mode blank-only
mybox $ gconftool-2 --direct \
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \
--type int \
--set /apps/gnome-screensaver/idle_delay 15