Manual:$wgAllowPrefChange

Details
Policies for how each preference is allowed to be changed, in the presence of external authentication. The keys are preference keys, e.g., 'password' or 'emailaddress' (see Preferences.php et al.). The value can be one of the following:


 * local - Allow changes to this pref through the wiki interface but only apply them locally (default).
 * semiglobal - Allow changes through the wiki interface and try to apply them to the foreign database, but continue on anyway if that fails.
 * global - Allow changes through the wiki interface, but only let them go through if they successfully update the foreign database.
 * message - Allow no local changes for linked accounts. Replace the change form with a message provided by the auth plugin, telling the user how to change the setting externally (maybe providing a link, etc.).  If the auth plugin provides no message for this preference, hide it entirely.

Note
Accounts that are not linked to an external account are never affected by this setting. You may want to look at  instead. $wgHiddenPrefs supersedes this option.