You can't prevent users from deleting the history.

You can lock settings for "Clear Recent History" (privacy.cpd.*) and "Clear history when Firefox closes" (privacy.clearOnShutdown.*), but you can still clear history items in the sidebar or the History Manager (Library).

Note that toolkit.telemetry.prompted is no longer a Boolean pref, but an integer as you can see on the about:config page

• lockPref("toolkit.telemetry.prompted",2);
• lockPref("toolkit.telemetry.rejected",true);

Is there a way to at least grey out the option to do so in the Privacy tab? Some security is better than none at all so if I can just get that done then I will be moving in a positive direction.

I can see that my setting are locked now in the about:config window. However, when I open the Options-->Privacy tab I can still change those settings. I have updated the file to include the below settings.

Test file:

// 
lockPref("privacy.item.history", false);
lockPref("privacy.item.downloads", false);
lockPref("privacy.item.cache", true);
lockPref("privacy.item.formdata", true);
lockPref("privacy.item.offlineApps", true);
lockPref("privacy.item.passwords", true);
lockPref("privacy.item.sessions", false);
lockPref("browser.preferences.privacy.selectedTabIndex", 4);
lockPref("browser.history_expire_days", 90);
lockPref("browser.history_expire_days_min", 90);
lockPref("browser.history_expire_sites", 40000);
lockPref("places.history.enabled", true);
lockPref("browser.privatebrowsing.autostart", false);
lockPref("privacy.clearOnShutdown.history", false);
lockPref("privacy.clearOnShutdown.formdata", false);
lockPref("privacy.clearOnShutdown.passwords", false);
lockPref("privacy.clearOnShutdown.downloads", false);
lockPref("privacy.clearOnShutdown.cookies", false);
lockPref("privacy.clearOnShutdown.cache", false);
lockPref("privacy.clearOnShutdown.sessions", false);
lockPref("privacy.clearOnShutdown.offlineApps", false);
lockPref("privacy.clearOnShutdown.siteSettings", false);
lockPref("privacy.cpd.history", false);
lockPref("privacy.cpd.formdata", false);
lockPref("privacy.cpd.passwords", false);
lockPref("privacy.cpd.downloads", false);
lockPref("privacy.cpd.cookies", false);
lockPref("privacy.cpd.cache", false);
lockPref("privacy.cpd.sessions", false);
lockPref("privacy.cpd.offlineApps", false);
lockPref("privacy.cpd.siteSettings", false);

Which settings are you trying to change in Option > Privacy and want to prevent others from changing?

You will only lock the settings in "Clear history when Firefox closes" and not the main settings.
You can have a look at the about:config page to see if there are prefs affected by toggling check-boxes.

I don't know what effect privacy.item.* prefs have.
The about:config page only shows one for privacy.item.cookies and the source code doesn't make it clear for me as well.

I'm not sure why you want to include items like sessions (Active Logins; basic authentication used by FTP servers) and the disk cache and offline application cache (offlineApps).
People should be able to clear some data in case of problems and clearing cache and the cookies should be one of them (we advise a lot of times to clear cache and cookies).
Clearing form and search data should also be possible unless you would disable saving both passwords and form/search data.

Note that only Tools > Options > Advanced has tabs, so browser.preferences.privacy.selectedTabIndex won't have any effect.
The browser.history_expire_days and browser.history_expire_days_min prefs are obsolete in current Firefox versions and only used in Firefox 3 versions.