You can set preferences for accounts which don't exist yet.
This means that if somebody has already signed in, changing the default settings will have no influence anymore on those account preferences; it will only work for not-yet-existing accounts.
To do this, you need to manually edit the registry.
You know that the preferences for a specific account are stored in the registry under:
HKEY_CURRENT_USER\Software\Patchou\Messenger Plus! Live\
<Windows Live ID>\Preferences
(if you didn't, you know now 
)
To change the default preferences, thus for accounts which don't exist yet, you need to set the preferences you want to overwrite in:
HKEY_CURRENT_USER\Software\Patchou\Messenger Plus! Live\
DefaultSettings\Preferences
For example, to disable Plus! logging of conversations, you must create a new DWORD key named 'EnableChatLog' in
"HKEY_CURRENT_USER\Software\Patchou\Messenger Plus! Live\DefaultSettings\Preferences" and set its value to 0.
The next time someone new will sign in on your Messenger, and thus a new default preference set will be generated for them by Plus!, their conversation logging will be disabled.
Note: removing an account (aka Windows Live ID) from the contact list's sign in list will not remove the Plus! preferences for that account! You need to manually remove its preference set in the registry.
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